This is false. If you turn off the "Messages in iCloud" feature then your messages are included in your regular iCloud backup which Apple has the keys to decrypt, as disclosed.
Of course iCloud backup is itself optional. But Apple gives you and the people you're messaging no other option for cloud backups. ADP actually encrypts your backups, but since it defaults to off your messages are almost certainly still readable by Apple thanks to the keys stored in other peoples' backups.
And of course ADP is off in the U.K., where I live. And iMessage sometimes randomly falls back to unencrypted SMS/MMS even when you ticked the checkbox disallowing this in System Settings.
> If you turn off the "Messages in iCloud" feature then your messages are included in your regular iCloud backup which Apple has the keys to decrypt, as disclosed.
No, if you do not use “Messages in iCloud” then your iMessage private key does not leave your device.
If you turn off Messages in iCloud then the messages are instead stored in your iCloud backup and encrypted "In transit & on server" with key storage by Apple, not just on your devices, as specified in the fourth row of the "Data categories and encryption" table in the Apple support article I linked. "In transit & on server" means not e2ee. That is, Apple can decrypt the messages at will without notice or consent.
If the messages were still protected by e2ee with key storage only on your devices then it would specify that in the table. Some other data types like keychain passwords and Memoji are in fact protected by e2ee even when ADP is not enabled, and the table reflects that. Messages do not fall in the category of e2ee without ADP.
Of course iCloud backup is itself optional. But Apple gives you and the people you're messaging no other option for cloud backups. ADP actually encrypts your backups, but since it defaults to off your messages are almost certainly still readable by Apple thanks to the keys stored in other peoples' backups.