[fh9302]

You either have to enable E2EE or disable both Messages in iCloud and device backups. Otherwise the device backups contain a copy of your messages.

[galad87]

The "Messages in iCloud" sync is end to end, so you can enable it and disable iCloud backup, or manually backup on your computer: https://support.apple.com/en-us/102651

[zchrykng]

Yeah, it is end to end encrypted, but the keys are part of your device's iCloud backup. So unless you turn on end to end encryption for that backup or disable it, Apple can access the keys required to decrypt the iMessage in iCloud messages.

[etchalon]

I believe the reason iMessages aren't protected with iCloud Backup is because they're stored decrypted in the SQLite database iMessage uses, chat.db.