diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -183,6 +183,9 @@
   <clang-tidy/checks/bugprone/use-after-move>` check to also cover constructor
   initializers.
 
+- Deprecated :doc:`cert-dcl21-cpp
+  <clang-tidy/checks/cert/dcl21-cpp>` check.
+
 - Deprecated check-local options `HeaderFileExtensions`
   in :doc:`google-build-namespaces
   <clang-tidy/checks/google/build-namespaces>` check.
diff --git a/clang-tools-extra/docs/clang-tidy/checks/cert/dcl21-cpp.rst b/clang-tools-extra/docs/clang-tidy/checks/cert/dcl21-cpp.rst
--- a/clang-tools-extra/docs/clang-tidy/checks/cert/dcl21-cpp.rst
+++ b/clang-tools-extra/docs/clang-tidy/checks/cert/dcl21-cpp.rst
@@ -3,6 +3,10 @@
 cert-dcl21-cpp
 ==============
 
+.. note::
+  This check is deprecated since it's no longer part of the CERT standard.
+  It will be removed in :program:`clang-tidy` version 19.
+
 This check flags postfix ``operator++`` and ``operator--`` declarations
 if the return type is not a const object. This also warns if the return type
 is a reference type.