You can have iCloud enabled for Messages, but if you have iCloud Backup on then the keys are stored in the backup. >Messages in iCloud also uses end-to-end enc

You can have iCloud enabled for Messages, but if you have iCloud Backup on then the keys are stored in the backup. >Messages in iCloud also uses end-to-end encryption. If you have iCloud Backup turned on, your backup includes a copy of the key protecting your Messages. This ensures you can recover your Messages if you lose access to iCloud Keychain and your trusted devices. When you turn off iCloud Backup, a new key is generated on your device to protect future messages and isn't stored by Apple.