diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index 68301b74d2f8b0..c993d58378708d 100644
--- a/Doc/library/sys.rst
+++ b/Doc/library/sys.rst
@@ -1235,6 +1235,9 @@ always available. Unless explicitly noted otherwise, all variables are read-only
 
    .. versionadded:: 3.13
 
+   .. impl-detail::
+
+      It is not guaranteed to exist in all implementations of Python.
 
 .. function:: is_finalizing()