Everyone you iMessage with will still be putting all of your conversations and attachments and iCloud message sync keys into non-e2ee backups from
their end, so turning this on won't accomplish much even if you know about it.
It doesn't matter as long as the person storing screenshots in iCloud turns it on.
> Everyone you iMessage with will still be putting all of your conversations and attachments and iCloud message sync keys into non-e2ee backups from their end
Weren't we talking about storing screenshots in iCloud photos?
But I still wouldn't trust Apple 100% : we know that they were among the companies silently cooperating with the NSA, and the potential for backdoors in their software isn't nil. (Whether you should consider this as a real threat depends on your circumstances of course.)
It's opt-in, so approximately nobody ever will.
Everyone you iMessage with will still be putting all of your conversations and attachments and iCloud message sync keys into non-e2ee backups from their end, so turning this on won't accomplish much even if you know about it.