Are messages included in iCloud System backup if iCloud Messages is disabled? 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?
I don't believe so - if you back up to a computer, everything is backed up as a full system image. When you back up to iCloud, it doesn't actually take a full system image (which in some cases could be hundreds of gigabytes in size), it establises what is backed up in iCloud, what you've selected as an option, and backs up your settings and app data, but not the whole system.
Which is why when you restore from an iCloud backup, you have to wait for your apps to download again, unlike if you restore from a computer backup where it's all immediately there.
If you want your iMessages backed up, you need to select the option.