With iCloud Backup, there are two ways data is stored: Apps Using iCloud and Backups.
The Backups portion is the equivalent of a full backup performed iTunes. Apps Using iCloud is app data written to the cloud storage, but accessible by apps and services outside of restoring your phone from a backup.
On iOS, it is possible to have your Photo Library backed up with the system backup while keeping iCloud Photo Library feature turned off.
When browsing the system backup options, Photo Library appears as an option but Messages does not. This leads me to believe that Messages could be excluded from the system backup.
If iCloud Messages as a service is turned off, are Messages data still included in the iCloud full system backup of the phone?