Deprecate support for source encodings other than UTF-8 (#13666)

doc/usage/restructuredtext/basics.rst

@@ -646,10 +646,11 @@ configurations:
 Source encoding
 ---------------
 
-Since the easiest way to include special characters like em dashes or copyright
-signs in reStructuredText is to directly write them as Unicode characters, one has to
-specify an encoding.  Sphinx assumes source files to be encoded in UTF-8 by
-default; you can change this with the :confval:`source_encoding` config value.
+Sphinx supports source files that are encoded in UTF-8.
+This means that the full range of Unicode__ characters may be used
+directly in reStructuredText.
+
+__ https://www.unicode.org/
 
 
 Gotchas

doc/usage/restructuredtext/directives.rst

@@ -971,7 +971,7 @@ __ https://pygments.org/docs/lexers
       :type: text
 
       Explicitly specify the encoding of the file.
-      This overwrites the default encoding (:confval:`source_encoding`).
+      This overwrites the default encoding (UTF-8).
       For example:
 
       .. code-block:: rst