> Yes, I know, defaults.

You gave the answer yourself. If the person you are writing with has E2EE enabled for message backups then just exclude that chat from backups on your end or enable E2EE for that chat. That means if you take the restore route without having access to the keys that conversation will not be restored.

You could even let people overwrite that setting. Preventing screenshots or copying has nothing to with this. Apple is claiming that message are end to end encrypted when in reality their defaults are set so that they are not. That is a lie and al they would need to do to change that is to change those defaults.