python
@@ -1898,6 +1898,12 @@ Argument groups
    Note that any arguments not in your user-defined groups will end up back
    in the usual "positional arguments" and "optional arguments" sections.
 
+   .. versionchanged:: 3.11
+    Calling :meth:`add_argument_group` on an argument group is deprecated.
+    This feature was never supported and does not always work correctly.
+    The function exists on the API by accident through inheritance and
+    will be removed in the future.
+
 
 Mutual exclusion
 ^^^^^^^^^^^^^^^^
@@ -1936,6 +1942,12 @@ Mutual exclusion
    *title* and *description* arguments of
    :meth:`~ArgumentParser.add_argument_group`.
 
+   .. versionchanged:: 3.11
+    Calling :meth:`add_argument_group` or :meth:`add_mutually_exclusive_group`
+    on a mutually exclusive group is deprecated. These features were never
+    supported and do not always work correctly. The functions exist on the
+    API by accident through inheritance and will be removed in the future.
+
 
 Parser defaults
 ^^^^^^^^^^^^^^^

@@ -89,6 +89,8 @@
 import re as _re
 import sys as _sys
 
+import warnings
+
 from gettext import gettext as _, ngettext
 
 SUPPRESS = '==SUPPRESS=='
@@ -1645,6 +1647,14 @@ def _remove_action(self, action):
         super(_ArgumentGroup, self)._remove_action(action)
         self._group_actions.remove(action)
 
+    def add_argument_group(self, *args, **kwargs):
+        warnings.warn(
+            "Nesting argument groups is deprecated.",
+            category=DeprecationWarning,
+            stacklevel=2
+        )
+        return super().add_argument_group(*args, **kwargs)
+
 
 class _MutuallyExclusiveGroup(_ArgumentGroup):
 
@@ -1665,6 +1675,14 @@ def _remove_action(self, action):
         self._container._remove_action(action)
         self._group_actions.remove(action)
 
+    def add_mutually_exclusive_group(self, *args, **kwargs):
+        warnings.warn(
+            "Nesting mutually exclusive groups is deprecated.",
+            category=DeprecationWarning,
+            stacklevel=2
+        )
+        return super().add_mutually_exclusive_group(*args, **kwargs)
+
 
 class ArgumentParser(_AttributeHolder, _ActionsContainer):
     """Object for parsing command line strings into Python objects.

@@ -9,6 +9,7 @@
 import tempfile
 import unittest
 import argparse
+import warnings
 
 from io import StringIO
 
@@ -2966,15 +2967,24 @@ def get_parser(self, required):
 
 class TestMutuallyExclusiveNested(MEMixin, TestCase):
 
+    # Nesting mutually exclusive groups is an undocumented feature
+    # that came about by accident through inheritance and has been
+    # the source of many bugs. It is deprecated and this test should
+    # eventually be removed along with it.
+
     def get_parser(self, required):
         parser = ErrorRaisingArgumentParser(prog='PROG')
         group = parser.add_mutually_exclusive_group(required=required)
         group.add_argument('-a')
         group.add_argument('-b')
-        group2 = group.add_mutually_exclusive_group(required=required)
+        with warnings.catch_warnings():
+            warnings.simplefilter('ignore', DeprecationWarning)
+            group2 = group.add_mutually_exclusive_group(required=required)
         group2.add_argument('-c')
         group2.add_argument('-d')
-        group3 = group2.add_mutually_exclusive_group(required=required)
+        with warnings.catch_warnings():
+            warnings.simplefilter('ignore', DeprecationWarning)
+            group3 = group2.add_mutually_exclusive_group(required=required)
         group3.add_argument('-e')
         group3.add_argument('-f')
         return parser

diff --git a/Misc/NEWS.d/next/Library/2021-12-15-19-24-54.bpo-22047.gBV4vT.rst b/Misc/NEWS.d/next/Library/2021-12-15-19-24-54.bpo-22047.gBV4vT.rst
@@ -0,0 +1,3 @@
+Calling :meth:`add_argument_group` on an argument group is deprecated. Calling :meth:`add_argument_group` or :meth:`add_mutually_exclusive_group` on a mutually exclusive group is deprecated.
+
+These features were never supported and do not always work correctly. The functions exist on the API by accident through inheritance and will be removed in the future.
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,3 @@
		Calling :meth:`add_argument_group` on an argument group is deprecated. Calling :meth:`add_argument_group` or :meth:`add_mutually_exclusive_group` on a mutually exclusive group is deprecated.

		These features were never supported and do not always work correctly. The functions exist on the API by accident through inheritance and will be removed in the future.