I am not sure if this is true. Yes, there is a ( obviously malfunctioning) rollback protection that can brick your device. BUT: I think what is important is the monthly Android security patches and not the main Android version in general. So I think you might be able to revert to the default OS if you have, say, LOS 20 (Android 13) March security patches installed. And there is a FPOS (Android 12) version with April security patches available…