diff --git a/app/assets/javascripts/discourse-markdown-it/src/setup.js b/app/assets/javascripts/discourse-markdown-it/src/setup.js
index 35d9ccf5a78..a26e40408b0 100644
--- a/app/assets/javascripts/discourse-markdown-it/src/setup.js
+++ b/app/assets/javascripts/discourse-markdown-it/src/setup.js
@@ -149,7 +149,6 @@ class Setup {
 
     discourse.limitedSiteSettings = {
       secureUploads: siteSettings.secure_uploads,
-      enableDiffhtmlPreview: siteSettings.enable_diffhtml_preview,
       traditionalMarkdownLinebreaks:
         siteSettings.traditional_markdown_linebreaks,
       enableMarkdownLinkify: siteSettings.enable_markdown_linkify,
diff --git a/app/assets/javascripts/discourse/app/components/composer-editor.js b/app/assets/javascripts/discourse/app/components/composer-editor.js
index d2887989cf3..aca1f7cd047 100644
--- a/app/assets/javascripts/discourse/app/components/composer-editor.js
+++ b/app/assets/javascripts/discourse/app/components/composer-editor.js
@@ -456,8 +456,8 @@ export default class ComposerEditor extends Component {
     $preview.scrollTop(desired + 50);
   }
 
-  _renderMentions(preview, unseen) {
-    unseen ||= linkSeenMentions(preview, this.siteSettings);
+  _renderMentions(preview) {
+    const unseen = linkSeenMentions(preview, this.siteSettings);
     if (unseen.length > 0) {
       this._renderUnseenMentions(preview, unseen);
     } else {
@@ -480,9 +480,9 @@ export default class ComposerEditor extends Component {
     });
   }
 
-  _renderHashtags(preview, unseen) {
+  _renderHashtags(preview) {
     const context = this.site.hashtag_configurations["topic-composer"];
-    unseen ||= linkSeenHashtagsInContext(context, preview);
+    const unseen = linkSeenHashtagsInContext(context, preview);
     if (unseen.length > 0) {
       this._renderUnseenHashtags(preview, unseen, context);
     }
@@ -896,15 +896,13 @@ export default class ComposerEditor extends Component {
   }
 
   @action
-  previewUpdated(preview, unseenMentions, unseenHashtags) {
-    this._renderMentions(preview, unseenMentions);
-    this._renderHashtags(preview, unseenHashtags);
+  previewUpdated(preview) {
+    this._renderMentions(preview);
+    this._renderHashtags(preview);
     this._refreshOneboxes(preview);
     this._expandShortUrls(preview);
 
-    if (!this.siteSettings.enable_diffhtml_preview) {
-      this._decorateCookedElement(preview);
-    }
+    this._decorateCookedElement(preview);
 
     this.composer.afterRefresh(preview);
   }
diff --git a/app/assets/javascripts/discourse/app/components/d-editor.hbs b/app/assets/javascripts/discourse/app/components/d-editor.hbs
index 8349bd32948..ae965622c54 100644
--- a/app/assets/javascripts/discourse/app/components/d-editor.hbs
+++ b/app/assets/javascripts/discourse/app/components/d-editor.hbs
@@ -75,9 +75,7 @@
     class="d-editor-preview-wrapper {{if this.forcePreview 'force-preview'}}"
   >
     <div class="d-editor-preview">
-      {{#unless this.siteSettings.enable_diffhtml_preview}}
-        {{html-safe this.preview}}
-      {{/unless}}
+      {{html-safe this.preview}}
     </div>
     <span class="d-editor-plugin">
       <PluginOutlet
diff --git a/app/assets/javascripts/discourse/app/components/d-editor.js b/app/assets/javascripts/discourse/app/components/d-editor.js
index 28a94be9f66..5cbca07b4d6 100644
--- a/app/assets/javascripts/discourse/app/components/d-editor.js
+++ b/app/assets/javascripts/discourse/app/components/d-editor.js
@@ -8,12 +8,10 @@ import { classNames } from "@ember-decorators/component";
 import { observes, on } from "@ember-decorators/object";
 import { emojiSearch, isSkinTonableEmoji } from "pretty-text/emoji";
 import { translations } from "pretty-text/emoji/data";
-import { resolveCachedShortUrls } from "pretty-text/upload-short-url";
 import { Promise } from "rsvp";
 import TextareaEditor from "discourse/components/composer/textarea-editor";
 import EmojiPickerDetached from "discourse/components/emoji-picker/detached";
 import InsertHyperlink from "discourse/components/modal/insert-hyperlink";
-import { ajax } from "discourse/lib/ajax";
 import { SKIP } from "discourse/lib/autocomplete";
 import Toolbar from "discourse/lib/composer/toolbar";
 import discourseDebounce from "discourse/lib/debounce";
@@ -22,11 +20,8 @@ import deprecated from "discourse/lib/deprecated";
 import { isTesting } from "discourse/lib/environment";
 import { getRegister } from "discourse/lib/get-owner";
 import { hashtagAutocompleteOptions } from "discourse/lib/hashtag-autocomplete";
-import { linkSeenHashtagsInContext } from "discourse/lib/hashtag-decorator";
 import { wantsNewWindow } from "discourse/lib/intercept-click";
 import { PLATFORM_KEY_MODIFIER } from "discourse/lib/keyboard-shortcuts";
-import { linkSeenMentions } from "discourse/lib/link-mentions";
-import { loadOneboxes } from "discourse/lib/load-oneboxes";
 import loadRichEditor from "discourse/lib/load-rich-editor";
 import { findRawTemplate } from "discourse/lib/raw-templates";
 import { emojiUrlFor, generateCookFunction } from "discourse/lib/text";
@@ -242,45 +237,6 @@ export default class DEditor extends Component {
 
     this.set("preview", cooked);
 
-    let unseenMentions, unseenHashtags;
-
-    if (this.siteSettings.enable_diffhtml_preview) {
-      const previewElement = this.element.querySelector(".d-editor-preview");
-      const cookedElement = previewElement.cloneNode(false);
-      cookedElement.innerHTML = cooked;
-
-      unseenMentions = linkSeenMentions(cookedElement, this.siteSettings);
-
-      unseenHashtags = linkSeenHashtagsInContext(
-        this.site.hashtag_configurations["topic-composer"],
-        cookedElement
-      );
-
-      loadOneboxes(
-        cookedElement,
-        ajax,
-        this.topicId,
-        this.categoryId,
-        this.siteSettings.max_oneboxes_per_post,
-        /* refresh */ false,
-        /* offline */ true
-      );
-
-      resolveCachedShortUrls(this.siteSettings, cookedElement);
-
-      // trigger all the "api.decorateCookedElement"
-      this.appEvents.trigger(
-        "decorate-non-stream-cooked-element",
-        cookedElement
-      );
-
-      (await import("morphlex")).morph(
-        previewElement,
-        cookedElement,
-        this.morphingOptions
-      );
-    }
-
     schedule("afterRender", () => {
       if (
         this._state !== "inDOM" ||
@@ -294,7 +250,7 @@ export default class DEditor extends Component {
       const previewElement = this.element.querySelector(".d-editor-preview");
 
       if (previewElement && this.previewUpdated) {
-        this.previewUpdated(previewElement, unseenMentions, unseenHashtags);
+        this.previewUpdated(previewElement);
       }
     });
   }
diff --git a/app/assets/javascripts/pretty-text/addon/upload-short-url.js b/app/assets/javascripts/pretty-text/addon/upload-short-url.js
index 81bd3810437..08854e4c5f7 100644
--- a/app/assets/javascripts/pretty-text/addon/upload-short-url.js
+++ b/app/assets/javascripts/pretty-text/addon/upload-short-url.js
@@ -159,31 +159,15 @@ function _loadCachedShortUrls(uploadElements, siteSettings, opts) {
 
         break;
       case "DIV":
-        if (siteSettings.enable_diffhtml_preview === true) {
-          retrieveCachedUrl(upload, siteSettings, "orig-src", opts, (url) => {
-            const videoHTML = `
-              <video width="100%" height="100%" preload="metadata" controls style="">
-                <source src="${url}">
-              </video>`;
-            upload.insertAdjacentHTML("beforeend", videoHTML);
-            upload.classList.add("video-container");
-          });
-        } else {
-          retrieveCachedUrl(
-            upload,
-            siteSettings,
-            "orig-src-id",
-            opts,
-            (url) => {
-              upload.style.backgroundImage = `url('${url}')`;
+        retrieveCachedUrl(upload, siteSettings, "orig-src-id", opts, (url) => {
+          upload.style.backgroundImage = `url('${url}')`;
 
-              const placeholderIcon = upload.querySelector(
-                ".placeholder-icon.video"
-              );
-              placeholderIcon.style.backgroundColor = "rgba(0, 0, 0, 0.3)";
-            }
+          const placeholderIcon = upload.querySelector(
+            ".placeholder-icon.video"
           );
-        }
+          placeholderIcon.style.backgroundColor = "rgba(0, 0, 0, 0.3)";
+        });
+
         break;
     }
   });
diff --git a/config/locales/server.en.yml b/config/locales/server.en.yml
index 48a6f8851d7..53c7f900a99 100644
--- a/config/locales/server.en.yml
+++ b/config/locales/server.en.yml
@@ -2596,7 +2596,6 @@ en:
     disable_watched_word_checking_in_user_fields: "disable watched word checking in user fields"
     watched_words_regular_expressions: "Allows the use of regular expressions for filtering words. If enabled, this feature groups sensitive words by their case-sensitivity. It then compiles all selected words into a single regular expression, adding word boundaries for regular watched words. Consequently, this regex-based filtering method adds an extra layer of control over content moderation by supporting more sophisticated word patterns. The setting also allows to easily substitute the original text with the replacement of choice."
 
-    enable_diffhtml_preview: "Experimental feature which uses diffHTML to sync preview instead of full re-render"
     enable_fast_edit: "Adds a button to the post selection menu to edit a small selection inline."
     old_post_notice_days: "The number of days after which a post notice is considered old. This visually differentiates it from newer notices on the site."
     new_user_notice_tl: "Minimum trust level required to see new user post notices."
diff --git a/config/site_settings.yml b/config/site_settings.yml
index 42c1c52db72..65e11909ebf 100644
--- a/config/site_settings.yml
+++ b/config/site_settings.yml
@@ -1310,10 +1310,6 @@ posting:
   watched_words_regular_expressions:
     client: true
     default: false
-  enable_diffhtml_preview:
-    hidden: true
-    default: false
-    client: true
   enable_fast_edit:
     default: true
     client: true
diff --git a/lib/onebox/engine/allowlisted_generic_onebox.rb b/lib/onebox/engine/allowlisted_generic_onebox.rb
index 235102591ae..a9f0be6d7aa 100644
--- a/lib/onebox/engine/allowlisted_generic_onebox.rb
+++ b/lib/onebox/engine/allowlisted_generic_onebox.rb
@@ -75,12 +75,8 @@ module Onebox
       def placeholder_html
         return article_html if (is_article? || force_article_html?)
         return image_html if is_image?
-        if !SiteSetting.enable_diffhtml_preview? && (is_video? || is_card?)
-          return Onebox::Helpers.video_placeholder_html
-        end
-        if !SiteSetting.enable_diffhtml_preview? && is_embedded?
-          return Onebox::Helpers.generic_placeholder_html
-        end
+        return Onebox::Helpers.video_placeholder_html if (is_video? || is_card?)
+        return Onebox::Helpers.generic_placeholder_html if is_embedded?
         to_html
       end
 
diff --git a/lib/onebox/engine/audio_onebox.rb b/lib/onebox/engine/audio_onebox.rb
index 8a41b100dd7..822923d8aec 100644
--- a/lib/onebox/engine/audio_onebox.rb
+++ b/lib/onebox/engine/audio_onebox.rb
@@ -23,7 +23,7 @@ module Onebox
       end
 
       def placeholder_html
-        SiteSetting.enable_diffhtml_preview ? to_html : ::Onebox::Helpers.audio_placeholder_html
+        ::Onebox::Helpers.audio_placeholder_html
       end
     end
   end
diff --git a/lib/onebox/engine/video_onebox.rb b/lib/onebox/engine/video_onebox.rb
index b0483bfbd40..3c810d7556c 100644
--- a/lib/onebox/engine/video_onebox.rb
+++ b/lib/onebox/engine/video_onebox.rb
@@ -29,7 +29,7 @@ module Onebox
       end
 
       def placeholder_html
-        SiteSetting.enable_diffhtml_preview ? to_html : ::Onebox::Helpers.video_placeholder_html
+        ::Onebox::Helpers.video_placeholder_html
       end
     end
   end
diff --git a/plugins/discourse-details/spec/system/morphed_preview_spec.rb b/plugins/discourse-details/spec/system/morphed_preview_spec.rb
deleted file mode 100644
index b379e4fe912..00000000000
--- a/plugins/discourse-details/spec/system/morphed_preview_spec.rb
+++ /dev/null
@@ -1,30 +0,0 @@
-# frozen_string_literal: true
-
-describe "Morphed Composer Preview", type: :system do
-  fab!(:user) { Fabricate(:user, refresh_auto_groups: true) }
-  let(:composer) { PageObjects::Components::Composer.new }
-
-  before do
-    SiteSetting.enable_diffhtml_preview = true
-    sign_in user
-    visit("/new-topic")
-  end
-
-  it "keeps details element open" do
-    composer.type_content <<~MD
-      [details=Velcro]
-      What a rip-off!
-      [/details]
-    MD
-
-    within(composer.preview) do
-      find("details").click
-      expect(page).to have_css("details[open]")
-    end
-
-    composer.move_cursor_after("rip-off!")
-    composer.type_content(" :person_facepalming:")
-
-    within(composer.preview) { expect(page).to have_css("details[open] img.emoji") }
-  end
-end
diff --git a/spec/system/composer/morphed_preview_spec.rb b/spec/system/composer/morphed_preview_spec.rb
deleted file mode 100644
index 702da839f95..00000000000
--- a/spec/system/composer/morphed_preview_spec.rb
+++ /dev/null
@@ -1,41 +0,0 @@
-# frozen_string_literal: true
-
-describe "Morphed Composer Preview", type: :system do
-  fab!(:user) { Fabricate(:user, username: "bob", refresh_auto_groups: true) }
-  let(:composer) { PageObjects::Components::Composer.new }
-
-  before do
-    SiteSetting.enable_diffhtml_preview = true
-    sign_in user
-    visit("/new-topic")
-  end
-
-  it "correctly morphs code blocks" do
-    composer.fill_content <<~MD
-      ```js
-      const = {
-        id: t.name,
-        text: t.name,
-        name: t.name,
-      ```
-    MD
-
-    within(composer.preview) { expect(find("code.lang-js")).to have_text("const = {") }
-
-    composer.move_cursor_after("const")
-    composer.type_content("ant")
-
-    within(composer.preview) { expect(find("code.lang-js")).to have_text("constant = {") }
-  end
-
-  it "correctly morphs mentions" do
-    composer.fill_content("@bob text")
-
-    within(composer.preview) { expect(find("a.mention")).to have_text("@bob") }
-
-    composer.select_all
-    composer.type_content("@system")
-
-    within(composer.preview) { expect(find("a.mention")).to have_text("@system") }
-  end
-end
diff --git a/spec/system/composer_uploads_spec.rb b/spec/system/composer_uploads_spec.rb
index bbee973b819..d3a752566a6 100644
--- a/spec/system/composer_uploads_spec.rb
+++ b/spec/system/composer_uploads_spec.rb
@@ -137,26 +137,6 @@ describe "Uploading files in the composer", type: :system do
       expect(composer).to have_no_in_progress_uploads
       expect(composer.preview).to have_css(".onebox-placeholder-container")
     end
-
-    it "shows video player in composer" do
-      SiteSetting.enable_diffhtml_preview = true
-
-      visit "/new-topic"
-      expect(composer).to be_opened
-      topic.fill_in_composer_title("Video upload test")
-
-      file_path_1 = file_from_fixtures("small.mp4", "media").path
-      attach_file(file_path_1) { composer.click_toolbar_button("upload") }
-
-      expect(composer).to have_no_in_progress_uploads
-      expect(composer.preview).to have_css(".video-container video")
-
-      expect(page).to have_css(
-        ".video-container video source[src]",
-        visible: false,
-        wait: Capybara.default_max_wait_time,
-      )
-    end
   end
 
   context "when multiple images are uploaded" do