With the release iOS 11.4, a new feature that was promised for iOS 11 is now available. Many times we have messages on multiple devices: iPad, iPhone, Mac, etc. When we read them on one everything is fine but deleting messages between those devices was a bit of a challenge. This new feature will sync messages across all iOS devices logged into the same iCloud account. The good news, too, is as one gets new devices, messages will appear as you left them on the previous device.
If you have not an install iOS 11.4, I highly recommend it. If anything for this feature but also for the many minor fine-tunings that come along with any iOS update.