diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 36d67c31ed0d..dee7be8e89fb 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -286,6 +286,15 @@ Two exceptions are `Protocol` and `runtime_checkable`: although
 these were added in Python 3.8, they can be used in stubs regardless
 of Python version.
 
+[PEP 688](https://www.python.org/dev/peps/pep-0688/), which is
+currently a draft, removes the implicit promotion of the
+`bytearray` and `memoryview` classes to `bytes`.
+Typeshed stubs should be written assuming that this proposal
+is accepted, so a parameter that accepts either `bytes` or
+`bytearray` should be typed as `bytes | bytearray`.
+Often one of the aliases from `_typeshed`, such as
+`_typeshed.ReadableBuffer`, can be used instead.
+
 ### What to include
 
 Stubs should include the complete interface (classes, functions,