diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -241,6 +241,9 @@
   suggest ``#else`` as an alternative. ``#elifdef`` and ``#elifndef`` are only
   suggested when in C2x or C++2b mode. Fixes
   `Issue 51598 <https://github.com/llvm/llvm-project/issues/51598>`_.
+- The ``-Wdeprecated`` diagnostic will now warn on out-of-line ``constexpr``
+  declarations downgraded to definitions in C++1z, in addition to the
+  existing warning on out-of-line ``const`` declarations.
 
 Non-comprehensive list of changes in this release
 -------------------------------------------------