iCloud backups (enabled by default) are not end-to-end encrypted.
So while it's technically E2E, in practice you get very little protection from it because it's broken by design.
I still use iMessage because of the user experience, but let's not be fooled by their misleading E2E claims; it's all just marketing BS.
E2E with centralized key management is primarily to protect you from casual/private threats (vendor employees, snoopers in your or your recipients network) not from legal authority.
It’s an active attack and can’t apply retroactively but within these constraints they can still do it.
What’s stopping some US government agency from forcing them to insert code that causes the Signal app to a indicate it is behaving correctly but isn’t?
And don’t say “laws”.
If your threat model includes advanced persistent threats all bets are off.