Messages in iCloud not implemented at present, coming in a future update to iOS 11
A feature touted for iOS 11 at the 2017 WWDC was Messages in iCloud — but it was removed during the beta process, and will return at some undisclosed point in the future.
The "Messages in iCloud" feature synchronized a user's Messages "stack" between devices seamlessly. With the feature set implemented, deleting a message on one logged-in device would strike it from all, plus only the newest messages would be stored on-device with the rest held in iCloud and loaded only when needed.
At present, there is no mention of the cloud-based Messages on the feature page for iOS 11.
AppleInsider reported that feature was removed from iOS 11 during the fifth beta release. Apple claimed at the time that the purged feature will ship in a future software update to iOS 11.
Even without the functionality, users can continue to receive and store messages on each device, and can continue to backup and restore messages using iCloud Backup.