diff --git a/CHANGES b/CHANGES
index 153dec87d..cdf469ed5 100644
--- a/CHANGES
+++ b/CHANGES
@@ -22,6 +22,7 @@ Deprecated
 * The ``suffix`` argument of ``env.doc2path()`` is deprecated.
 * The string style ``base`` argument of ``env.doc2path()`` is deprecated.
 * ``sphinx.application.Sphinx._setting_up_extension``
+* ``sphinx.ext.config.check_unicode()``
 * ``sphinx.ext.doctest.doctest_encode()``
 * ``sphinx.testing.util.remove_unicode_literal()``
 * ``sphinx.util.get_matching_docs()`` is deprecated
diff --git a/doc/extdev/index.rst b/doc/extdev/index.rst
index f6c813b0d..d16d792d4 100644
--- a/doc/extdev/index.rst
+++ b/doc/extdev/index.rst
@@ -133,6 +133,11 @@ The following is a list of deprecated interfaces.
      - 4.0
      - ``os.path.join()``
 
+   * - ``sphinx.ext.config.check_unicode()``
+     - 2.0
+     - 4.0
+     - N/A
+
    * - ``sphinx.ext.doctest.doctest_encode()``
      - 2.0
      - 4.0
diff --git a/sphinx/application.py b/sphinx/application.py
index 071d0cb33..bec79b7ff 100644
--- a/sphinx/application.py
+++ b/sphinx/application.py
@@ -25,7 +25,7 @@ from docutils.parsers.rst import Directive, directives, roles
 
 import sphinx
 from sphinx import package_dir, locale
-from sphinx.config import Config, check_unicode
+from sphinx.config import Config
 from sphinx.config import CONFIG_FILENAME  # NOQA # for compatibility (RemovedInSphinx30)
 from sphinx.deprecation import (
     RemovedInSphinx30Warning, RemovedInSphinx40Warning
@@ -200,7 +200,6 @@ class Sphinx:
             self.config = Config({}, confoverrides or {})
         else:
             self.config = Config.read(self.confdir, confoverrides or {}, self.tags)
-            check_unicode(self.config)
 
         # initialize some limited config variables before initialize i18n and loading
         # extensions
diff --git a/sphinx/cmd/build.py b/sphinx/cmd/build.py
index 35176c7fc..fe167413a 100644
--- a/sphinx/cmd/build.py
+++ b/sphinx/cmd/build.py
@@ -18,7 +18,7 @@ import sys
 import traceback
 
 from docutils.utils import SystemMessage
-from six import text_type, binary_type
+from six import text_type
 
 import sphinx.locale
 from sphinx import __display_version__, package_dir
@@ -230,13 +230,6 @@ def build_main(argv=sys.argv[1:]):  # type: ignore
     if missing_files:
         parser.error(__('cannot find files %r') % missing_files)
 
-    # likely encoding used for command-line arguments
-    try:
-        locale = __import__('locale')  # due to submodule of the same name
-        likely_encoding = locale.getpreferredencoding()
-    except Exception:
-        likely_encoding = None
-
     if args.force_all and filenames:
         parser.error(__('cannot combine -a option and filenames'))
 
@@ -268,11 +261,6 @@ def build_main(argv=sys.argv[1:]):  # type: ignore
             key, val = val.split('=', 1)
         except ValueError:
             parser.error(__('-D option argument must be in the form name=value'))
-        if likely_encoding and isinstance(val, binary_type):
-            try:
-                val = val.decode(likely_encoding)
-            except UnicodeError:
-                pass
         confoverrides[key] = val
 
     for val in args.htmldefine:
@@ -283,11 +271,7 @@ def build_main(argv=sys.argv[1:]):  # type: ignore
         try:
             val = int(val)
         except ValueError:
-            if likely_encoding and isinstance(val, binary_type):
-                try:
-                    val = val.decode(likely_encoding)
-                except UnicodeError:
-                    pass
+            pass
         confoverrides['html_context.%s' % key] = val
 
     if args.nitpicky:
diff --git a/sphinx/cmd/quickstart.py b/sphinx/cmd/quickstart.py
index 94f829e13..934f47b2a 100644
--- a/sphinx/cmd/quickstart.py
+++ b/sphinx/cmd/quickstart.py
@@ -35,7 +35,7 @@ except ImportError:
     USE_LIBEDIT = False
 
 from docutils.utils import column_width
-from six import text_type, binary_type
+from six import text_type
 
 import sphinx.locale
 from sphinx import __display_version__, package_dir
@@ -644,11 +644,6 @@ def main(argv=sys.argv[1:]):
         print('[Interrupted.]')
         return 130  # 128 + SIGINT
 
-    # decode values in d if value is a Python string literal
-    for key, value in d.items():
-        if isinstance(value, binary_type):
-            d[key] = term_decode(value)
-
     # handle use of CSV-style extension values
     d.setdefault('extensions', [])
     for ext in d['extensions'][:]:
diff --git a/sphinx/config.py b/sphinx/config.py
index 45ba9f7d9..f5a7106d7 100644
--- a/sphinx/config.py
+++ b/sphinx/config.py
@@ -17,9 +17,9 @@ from collections import OrderedDict
 from os import path, getenv
 from typing import Any, NamedTuple, Union
 
-from six import string_types, binary_type, text_type, integer_types
+from six import string_types, text_type, integer_types
 
-from sphinx.deprecation import RemovedInSphinx30Warning
+from sphinx.deprecation import RemovedInSphinx30Warning, RemovedInSphinx40Warning
 from sphinx.errors import ConfigError, ExtensionError
 from sphinx.locale import _, __
 from sphinx.util import logging
@@ -488,10 +488,13 @@ def check_unicode(config):
     """check all string values for non-ASCII characters in bytestrings,
     since that can result in UnicodeErrors all over the place
     """
+    warnings.warn('sphinx.config.check_unicode() is deprecated.',
+                  RemovedInSphinx40Warning)
+
     nonascii_re = re.compile(br'[\x80-\xff]')
 
     for name, value in config._raw_config.items():
-        if isinstance(value, binary_type) and nonascii_re.search(value):
+        if isinstance(value, bytes) and nonascii_re.search(value):
             logger.warning(__('the config value %r is set to a string with non-ASCII '
                               'characters; this can lead to Unicode errors occurring. '
                               'Please use Unicode strings, e.g. %r.'), name, u'Content')
diff --git a/sphinx/ext/apidoc.py b/sphinx/ext/apidoc.py
index 31cdb5855..36073298f 100644
--- a/sphinx/ext/apidoc.py
+++ b/sphinx/ext/apidoc.py
@@ -25,8 +25,6 @@ import sys
 from fnmatch import fnmatch
 from os import path
 
-from six import binary_type
-
 import sphinx.locale
 from sphinx import __display_version__, package_dir
 from sphinx.cmd.quickstart import EXTENSIONS
@@ -444,15 +442,6 @@ def main(argv=sys.argv[1:]):
         if args.extensions:
             d['extensions'].extend(args.extensions)
 
-        if isinstance(args.header, binary_type):
-            d['project'] = d['project'].decode('utf-8')
-        if isinstance(args.author, binary_type):
-            d['author'] = d['author'].decode('utf-8')
-        if isinstance(args.version, binary_type):
-            d['version'] = d['version'].decode('utf-8')
-        if isinstance(args.release, binary_type):
-            d['release'] = d['release'].decode('utf-8')
-
         if not args.dryrun:
             qs.generate(d, silent=True, overwrite=args.force)
     elif args.tocfile:
diff --git a/sphinx/ext/doctest.py b/sphinx/ext/doctest.py
index b2244d032..0f706ae87 100644
--- a/sphinx/ext/doctest.py
+++ b/sphinx/ext/doctest.py
@@ -22,13 +22,13 @@ from docutils import nodes
 from docutils.parsers.rst import directives
 from packaging.specifiers import SpecifierSet, InvalidSpecifier
 from packaging.version import Version
-from six import StringIO, binary_type
+from six import StringIO
 
 import sphinx
 from sphinx.builders import Builder
 from sphinx.deprecation import RemovedInSphinx40Warning
 from sphinx.locale import __
-from sphinx.util import force_decode, logging
+from sphinx.util import logging
 from sphinx.util.console import bold  # type: ignore
 from sphinx.util.docutils import SphinxDirective
 from sphinx.util.nodes import set_source_info
@@ -330,8 +330,6 @@ class DocTestBuilder(Builder):
             logger.warning(text)
         else:
             logger.info(text, nonl=True)
-        if isinstance(text, binary_type):
-            text = force_decode(text, None)
         self.outfile.write(text)
 
     def get_target_uri(self, docname, typ=None):
diff --git a/sphinx/util/__init__.py b/sphinx/util/__init__.py
index 82218e335..e11ae1855 100644
--- a/sphinx/util/__init__.py
+++ b/sphinx/util/__init__.py
@@ -28,7 +28,7 @@ from time import mktime, strptime
 from urllib.parse import urlsplit, urlunsplit, quote_plus, parse_qsl, urlencode
 
 from docutils.utils import relative_path
-from six import text_type, binary_type
+from six import text_type
 
 from sphinx.deprecation import RemovedInSphinx30Warning, RemovedInSphinx40Warning
 from sphinx.errors import PycodeError, SphinxParallelError, ExtensionError
@@ -458,7 +458,7 @@ def parselinenos(spec, total):
 def force_decode(string, encoding):
     # type: (unicode, unicode) -> unicode
     """Forcibly get a unicode string out of a bytestring."""
-    if isinstance(string, binary_type):
+    if isinstance(string, bytes):
         try:
             if encoding:
                 string = string.decode(encoding)
diff --git a/sphinx/util/inspect.py b/sphinx/util/inspect.py
index 82814e00d..885a7b929 100644
--- a/sphinx/util/inspect.py
+++ b/sphinx/util/inspect.py
@@ -18,9 +18,8 @@ import sys
 import typing
 from functools import partial
 
-from six import StringIO, binary_type, string_types
+from six import StringIO, string_types
 
-from sphinx.util import force_decode
 from sphinx.util import logging
 from sphinx.util.pycompat import NoneType
 
@@ -249,8 +248,6 @@ def object_description(object):
         s = repr(object)
     except Exception:
         raise ValueError
-    if isinstance(s, binary_type):
-        s = force_decode(s, None)  # type: ignore
     # Strip non-deterministic memory addresses such as
     # ``<__main__.A at 0x7f68cb685710>``
     s = memory_address_re.sub('', s)