On my iOS 11.x device I've enabled iMessages in iCloud. When I back up my device locally via iTunes, are my iMessages still included in that backup?
Your messages will be backed up if stored on the device you are backing up. For example, if you are backing up A and the messages are stored on B, it won't back it up.
A backup is recommended as if you loose your messages on all devices they will be lost.
Messages in iCloud are updated automatically, so you always have the same view everywhere you use iMessage. When you delete a message, photo, or conversation on one device, it’s removed from all of your devices. And since all of your attachments are stored in iCloud, you can save space on your device.
Your messages will be backed up to iCloud if you back up the device that has the messages! For example....you have important messages on iPhone A and you just bought a new iPhone (iPhone B). You want the messages from iPhone A to be put on iPhone B. In order to do this you need to do an iCloud backup on iPhone A and then do a “Restore from iCloud Backup” on iPhone B! Does this make sense?
EDIT: I didn’t realize you were talking about an iTunes backup, but it is essentially the exact same concept, just with iTunes instead of iCloud! My mistake!
Best way to check and make sure - crack open the iTunes backup. iPhone Backup Browser is the original, now abandonware, but you have other options, like iBackup Viewer, iPhone Backup Viewer, and the good folks at iMazing do a utility. The utility would you usually present backup files in a human-readable way, so you'd know where messages are. On older iOS (and on newer ones to some extent) simply moving SMS.db from one device to the other was enough to get your messages