diff --git a/doc/api/util.md b/doc/api/util.md
index d02e8e9299a126..e77502c070da1e 100644
--- a/doc/api/util.md
+++ b/doc/api/util.md
@@ -406,6 +406,8 @@ changes:
     description: The `constructor` parameter can refer to an ES6 class now.
 -->
 
+> Stability: 3 - Legacy: It is recommended to use ES6 class `extends` instead.
+
 * `constructor` {Function}
 * `superConstructor` {Function}