The end-to-end encryption means very little if you have your messages backed up in iCloud because the encryption keys are also stored with the messages in iCloud according to this FBI document. If that’s the case, Apple can definitely read your messages as well as (obviously) any government agency who can make a legal request to Apple.
Small extract:
“Conceptually, Advanced Data Protection is simple: All CloudKit Service keys that were generated on device and later uploaded to the available-after-authentication iCloud Hardware Security Modules (HSMs) in Apple data centers are deleted from those HSMs and instead kept entirely within the account’s iCloud Keychain protection domain. They are handled like the existing end-to-end encrypted service keys, which means Apple can no longer read or access these keys.”
@bender@twtxt.net That’s great, actually, but it’s a shame you have to opt in to it.
@mckinley@twtxt.net it is opt-in because all your devices logged into the same iCloud account need to be at a compatible level. You also have to have a designated recovery account member which, obviously, you need to manually add.