diff --git a/importlib_metadata/_adapters.py b/importlib_metadata/_adapters.py
index 120e43a0..6223263e 100644
--- a/importlib_metadata/_adapters.py
+++ b/importlib_metadata/_adapters.py
@@ -1,20 +1,8 @@
-import functools
-import warnings
 import re
 import textwrap
 import email.message
 
 from ._text import FoldedCase
-from ._compat import pypy_partial
-
-
-# Do not remove prior to 2024-01-01 or Python 3.14
-_warn = functools.partial(
-    warnings.warn,
-    "Implicit None on return values is deprecated and will raise KeyErrors.",
-    DeprecationWarning,
-    stacklevel=pypy_partial(2),
-)
 
 
 class Message(email.message.Message):
@@ -53,12 +41,17 @@ def __iter__(self):
 
     def __getitem__(self, item):
         """
-        Warn users that a ``KeyError`` can be expected when a
-        missing key is supplied. Ref python/importlib_metadata#371.
+        Override parent behavior to typical dict behavior.
+
+        ``email.message.Message`` will emit None values for missing
+        keys. Typical mappings, including this ``Message``, will raise
+        a key error for missing keys.
+
+        Ref python/importlib_metadata#371.
         """
         res = super().__getitem__(item)
         if res is None:
-            _warn()
+            raise KeyError(item)
         return res
 
     def _repair_headers(self):
diff --git a/newsfragments/371.removal.rst b/newsfragments/371.removal.rst
new file mode 100644
index 00000000..7aca2d4f
--- /dev/null
+++ b/newsfragments/371.removal.rst
@@ -0,0 +1 @@
+Message.__getitem__ now raises a KeyError on missing keys.
\ No newline at end of file
diff --git a/tests/test_api.py b/tests/test_api.py
index fc20987e..7ce0cd64 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -1,9 +1,7 @@
 import re
 import textwrap
 import unittest
-import warnings
 import importlib
-import contextlib
 
 from . import fixtures
 from importlib_metadata import (
@@ -18,13 +16,6 @@
 )
 
 
-@contextlib.contextmanager
-def suppress_known_deprecation():
-    with warnings.catch_warnings(record=True) as ctx:
-        warnings.simplefilter('default', category=DeprecationWarning)
-        yield ctx
-
-
 class APITests(
     fixtures.EggInfoPkg,
     fixtures.EggInfoPkgPipInstalledNoToplevel,
@@ -157,13 +148,13 @@ def test_importlib_metadata_version(self):
         resolved = version('importlib-metadata')
         assert re.match(self.version_pattern, resolved)
 
-    def test_missing_key_legacy(self):
+    def test_missing_key(self):
         """
-        Requesting a missing key will still return None, but warn.
+        Requesting a missing key raises KeyError.
         """
         md = metadata('distinfo-pkg')
-        with suppress_known_deprecation():
-            assert md['does-not-exist'] is None
+        with self.assertRaises(KeyError):
+            md['does-not-exist']
 
     def test_get_key(self):
         """