If the sender or recipient has iCloud backup enabled then by default (i.e. without ADP) Apple can read the entire iMessage conversation. And they routinely do, at the request of law enforcement. Since Apple does not allow default-secure alternative cloud backup solutions to exist, it is almost certain that a large majority of iMessage conversations are compromised in this way (with no notification to sender or recipient).

Apple deliberately makes this non-obvious, but it is disclosed here: https://support.apple.com/en-us/102651

> Messages in iCloud is end-to-end encrypted when iCloud Backup is disabled. When iCloud Backup is enabled, your backup includes a copy of the Messages in iCloud encryption key to help you recover your data. If you turn off iCloud Backup, a new key is generated on your device to protect future Messages in iCloud. This key is end-to-end encrypted between your devices and isnʼt stored by Apple

And is the backup end-to-end encrypted? No, not by default, as disclosed on the same page. It is encrypted "In transit & on server" with keys stored by Apple, which means Apple can decrypt it. And they do, as mentioned earlier, for purposes other than "to help you recover your data". The non-default Advanced Data Protection feature is required to get end-to-end encryption of the backup.

Note that Google's equivalent Android backup feature has been end-to-end encrypted by default for many, many years. Plus, alternative backup solutions are allowed to exist on Android.