Juli Clover, writing for MacRumors:
As of iOS 11.3, Messages in iCloud is back and is available for “testing and evaluation purposes,” marking the official return of the feature. According to Apple’s release notes, once iOS 11.3 beta 1 is installed, Messages will prompt users to turn on Messages in the Cloud when it is first launched.
Messages in the Cloud is automatically enabled for users who have two-factor authentication and iCloud Backup enabled, says Apple.
One of the advantages with Messages on iCloud is that, if you want to keep your old messages around, they won’t be taking up space on your devices, and they’ll only count once against your iCloud storage limit (as opposed to now, where old messages are stored in the backups of each device you use).