If Android 11 becomes unstable, revert using (not fastboot). Download the correct ops file for your model. MSM bypasses all partition locks by rewriting at the Qualcomm Sahara protocol level.
This is the recommended starting point for most users. It's an official process that uses a signed ZIP file from OnePlus, which is a rollback package designed to take your phone from Android 12 or 13 back to Android 11. Even though it's official, this process will completely wipe your data. downgrade oneplus 9 to android 11 exclusive