squash: adjust guideline

Author: LiviaMedeiros

test/common/README.md

@@ -308,8 +308,7 @@ If `target` is an Object, returns a proxy object that triggers
 an `AssertionError` on mutation attempt, including mutation of deeply nested
 Objects. Otherwise, it returns `target` directly.
 
-Use of this function is encouraged for relevant regression tests and for new
-APIs that work with user-provided objects.
+Use of this function is encouraged for relevant regression tests.
 
 ```mjs
 import { open } from 'node:fs/promises';