diff --git a/doc/api/next_api_changes/deprecations/18817-BGB.rst b/doc/api/next_api_changes/deprecations/18817-BGB.rst
new file mode 100644
index 000000000000..3088cd5d0db5
--- /dev/null
+++ b/doc/api/next_api_changes/deprecations/18817-BGB.rst
@@ -0,0 +1,3 @@
+Line2D and Patch no longer duplicate ``validJoin`` and ``validCap``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Validation of joinstyle and capstyles is now centralized in ``rcsetup``.
diff --git a/doc/conf.py b/doc/conf.py
index da3c981ec693..f220e1f28e37 100644
--- a/doc/conf.py
+++ b/doc/conf.py
@@ -13,8 +13,10 @@
 import shutil
 import subprocess
 import sys
+import warnings
 
 import matplotlib
+from matplotlib._api import MatplotlibDeprecationWarning
 import sphinx
 
 from datetime import datetime
@@ -109,6 +111,11 @@ def _check_dependencies():
 autodoc_docstring_signature = True
 autodoc_default_options = {'members': None, 'undoc-members': None}
 
+# make sure to ignore warnings that stem from simply inspecting deprecated
+# class-level attributes
+warnings.filterwarnings('ignore', category=MatplotlibDeprecationWarning,
+                        module='sphinx.util.inspect')
+
 # missing-references names matches sphinx>=3 behavior, so we can't be nitpicky
 # for older sphinxes.
 nitpicky = sphinx.version_info >= (3,)
diff --git a/lib/matplotlib/_api/__init__.py b/lib/matplotlib/_api/__init__.py
index 96e7d03c33f0..fba2d2e7e1dc 100644
--- a/lib/matplotlib/_api/__init__.py
+++ b/lib/matplotlib/_api/__init__.py
@@ -19,10 +19,44 @@
     deprecated, warn_deprecated,
     _rename_parameter, _delete_parameter, _make_keyword_only,
     _deprecate_method_override, _deprecate_privatize_attribute,
-    _suppress_matplotlib_deprecation_warning,
+    suppress_matplotlib_deprecation_warning,
     MatplotlibDeprecationWarning)
 
 
+class classproperty:
+    """
+    Like `property`, but also triggers on access via the class, and it is the
+    *class* that's passed as argument.
+
+    Examples
+    --------
+    ::
+
+        class C:
+            @classproperty
+            def foo(cls):
+                return cls.__name__
+
+        assert C.foo == "C"
+    """
+
+    def __init__(self, fget, fset=None, fdel=None, doc=None):
+        self._fget = fget
+        if fset is not None or fdel is not None:
+            raise ValueError('classproperty only implements fget.')
+        self.fset = fset
+        self.fdel = fdel
+        # docs are ignored for now
+        self._doc = doc
+
+    def __get__(self, instance, owner):
+        return self._fget(owner)
+
+    @property
+    def fget(self):
+        return self._fget
+
+
 def check_in_list(_values, *, _print_supported_values=True, **kwargs):
     """
     For each *key, value* pair in *kwargs*, check that *value* is in *_values*.
diff --git a/lib/matplotlib/_api/deprecation.py b/lib/matplotlib/_api/deprecation.py
index 542cc25ae627..f29806261ab9 100644
--- a/lib/matplotlib/_api/deprecation.py
+++ b/lib/matplotlib/_api/deprecation.py
@@ -186,6 +186,7 @@ def the_function_to_deprecate():
 
     def deprecate(obj, message=message, name=name, alternative=alternative,
                   pending=pending, obj_type=obj_type, addendum=addendum):
+        from matplotlib._api import classproperty
 
         if isinstance(obj, type):
             if obj_type is None:
@@ -202,15 +203,16 @@ def finalize(wrapper, new_doc):
                 obj.__init__ = functools.wraps(obj.__init__)(wrapper)
                 return obj
 
-        elif isinstance(obj, property):
+        elif isinstance(obj, (property, classproperty)):
             obj_type = "attribute"
             func = None
             name = name or obj.fget.__name__
             old_doc = obj.__doc__
 
-            class _deprecated_property(property):
+            class _deprecated_property(type(obj)):
                 def __get__(self, instance, owner):
-                    if instance is not None:
+                    if instance is not None or owner is not None \
+                            and isinstance(self, classproperty):
                         emit_warning()
                     return super().__get__(instance, owner)
 
@@ -518,7 +520,7 @@ def empty_with_docstring(): """doc"""
 
 
 @contextlib.contextmanager
-def _suppress_matplotlib_deprecation_warning():
+def suppress_matplotlib_deprecation_warning():
     with warnings.catch_warnings():
         warnings.simplefilter("ignore", MatplotlibDeprecationWarning)
         yield
diff --git a/lib/matplotlib/cbook/__init__.py b/lib/matplotlib/cbook/__init__.py
index cbfe9db794f1..caa872424535 100644
--- a/lib/matplotlib/cbook/__init__.py
+++ b/lib/matplotlib/cbook/__init__.py
@@ -29,11 +29,13 @@
 
 import matplotlib
 from matplotlib import _c_internal_utils
-from matplotlib._api import warn_external as _warn_external
+from matplotlib._api import (
+    warn_external as _warn_external, classproperty as _classproperty)
 from matplotlib._api.deprecation import (
     deprecated, warn_deprecated,
     _rename_parameter, _delete_parameter, _make_keyword_only,
     _deprecate_method_override, _deprecate_privatize_attribute,
+    suppress_matplotlib_deprecation_warning as
     _suppress_matplotlib_deprecation_warning,
     MatplotlibDeprecationWarning, mplDeprecation)
 
@@ -2262,30 +2264,6 @@ def type_name(tp):
                     type_name(type(v))))
 
 
-class _classproperty:
-    """
-    Like `property`, but also triggers on access via the class, and it is the
-    *class* that's passed as argument.
-
-    Examples
-    --------
-    ::
-
-        class C:
-            @classproperty
-            def foo(cls):
-                return cls.__name__
-
-        assert C.foo == "C"
-    """
-
-    def __init__(self, fget):
-        self._fget = fget
-
-    def __get__(self, instance, owner):
-        return self._fget(owner)
-
-
 def _backend_module_name(name):
     """
     Convert a backend name (either a standard backend -- "Agg", "TkAgg", ... --
diff --git a/lib/matplotlib/lines.py b/lib/matplotlib/lines.py
index 1aea828908e6..aac40606e015 100644
--- a/lib/matplotlib/lines.py
+++ b/lib/matplotlib/lines.py
@@ -3,7 +3,6 @@
 colors.
 """
 
-# TODO: expose cap and join style attrs
 from numbers import Integral, Number, Real
 import logging
 
@@ -251,8 +250,16 @@ class Line2D(Artist):
     fillStyles = MarkerStyle.fillstyles
 
     zorder = 2
-    validCap = ('butt', 'round', 'projecting')
-    validJoin = ('miter', 'round', 'bevel')
+
+    @_api.deprecated("3.4")
+    @_api.classproperty
+    def validCap(cls):
+        return ('butt', 'round', 'projecting')
+
+    @_api.deprecated("3.4")
+    @_api.classproperty
+    def validJoin(cls):
+        return ('miter', 'round', 'bevel')
 
     def __str__(self):
         if self._label != "":
diff --git a/lib/matplotlib/patches.py b/lib/matplotlib/patches.py
index d91ff5dcee85..22d5da2c2ff9 100644
--- a/lib/matplotlib/patches.py
+++ b/lib/matplotlib/patches.py
@@ -33,8 +33,18 @@ class Patch(artist.Artist):
     are *None*, they default to their rc params setting.
     """
     zorder = 1
-    validCap = mlines.Line2D.validCap
-    validJoin = mlines.Line2D.validJoin
+
+    @_api.deprecated("3.4")
+    @_api.classproperty
+    def validCap(cls):
+        with _api.suppress_matplotlib_deprecation_warning():
+            return mlines.Line2D.validCap
+
+    @_api.deprecated("3.4")
+    @_api.classproperty
+    def validJoin(cls):
+        with _api.suppress_matplotlib_deprecation_warning():
+            return mlines.Line2D.validJoin
 
     # Whether to draw an edge by default.  Set on a
     # subclass-by-subclass basis.
diff --git a/lib/matplotlib/tests/test_api.py b/lib/matplotlib/tests/test_api.py
index be2d80bb4244..5df3e4f75a2d 100644
--- a/lib/matplotlib/tests/test_api.py
+++ b/lib/matplotlib/tests/test_api.py
@@ -19,3 +19,16 @@ def test_check_shape(target, test_shape):
     data = np.zeros(test_shape)
     with pytest.raises(ValueError, match=error_pattern):
         _api.check_shape(target, aardvark=data)
+
+
+def test_classproperty_deprecation():
+    class A:
+        @_api.deprecated("0.0.0")
+        @_api.classproperty
+        def f(cls):
+            pass
+    with pytest.warns(_api.MatplotlibDeprecationWarning):
+        A.f
+    with pytest.warns(_api.MatplotlibDeprecationWarning):
+        a = A()
+        a.f