Messages in iCloud behaviour
When one enables Messages in iCloud, iOS alerts you that the setting on the device will be changed to Forever. Forever is the only setting available when Messages in iCloud is enabled.



The goal of keeping all Messages in iCloud is that each device should sync up each and every message they have (some will have forever and all messages they received, some will have gaps and some will have duplicate messages).

You'll no longer back up any messages when you do an iCloud Backup. All participating devices will show the exact same set of messages.


  When you use Messages in iCloud […] your content is automatically stored in iCloud. That means they're not included in your iCloud Backup.