Back at WWDC 2017, Apple announced that the sync feature would launch with iOS 11.
"Now with your iMessages in iCloud, when you sign in to another new device, all of your conversations are automatically synchronized. In fact, they stay in sync. If you want to delete an embarrassing message that you don't want in your transcript. It goes away everywhere, which is kind of nice. This of course allows us to optimize your device storage because with your messages in the cloud we only need to keep your most recent messages cached on the device and so that makes for smaller and faster backups. It is available for iOS and the Mac and of course your messages remain end to end encrypted."
The feature did appear in the first few betas of iOS 11 but was pulled before the final release. Let's hope it makes it through beta testing this time around!
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