Add release note

libcxx/docs/ReleaseNotes/19.rst

@@ -130,6 +130,12 @@ Deprecations and Removals
   `std-allocator-const <https://clang.llvm.org/extra/clang-tidy/checks/portability/std-allocator-const.html>`
   enabled.
 
+- When configuring libc++ with localization or threads disabled, the library no longer emits an error when
+  trying to ``#include <locale>`` and other such headers. Instead, those headers have no content. This is
+  consistent with the behavior for all other libc++ carve-outs like filesystem, wide characters, a source
+  of randomness, and others. Users that were checking whether including a header would fail (e.g. via a script
+  or CMake's ``try_compile`` will experience a change in behavior).
+
 
 Upcoming Deprecations and Removals
 ----------------------------------