Here's everything that is new in this release. This update brings to the Mac the Messages in iCloud feature, which already arrived on iOS with the iOS 10.4 update (see "iOS 11.4, tvOS 11.4, HomePod 11.4, and watchOS 4.3.1 Finally Bring Airplay 2 and Messages in iCloud", 29 May 2018).
Apple released macOS 10.13.5 today, an update that brings the long-awaited Messages in iCloud feature to Mac users. This update features "Messages in iCloud" which are updated automatically, so you always have the same view everywhere you use iMessage. Be sure to backup your data if you are going ahead otherwise.
This update also improves the "stability, performance, and security of your Mac, and is recommended for all users", explains Apple. The feature allows you to store communications you receive and send within the Messages app-plus associated file attachments-in iCloud to save space on your local device. The process may take a little while, so be patient and don't turn off your Mac manually if you don't have to. Also, since the completion of the update is dependent on a restart, therefore save all your work ahead of time. It was one day after the release of the 10.13.6 update for developers. This means that you'll notice less app crashes and hopefully a smoother day-to-day experience.
We're very glad to see the inclusion of Messages in iCloud in this release since it makes managing conversations far more easier. Delete one thread on your iPhone or iPad and it will be gone from your desktop as well, the way it should be.
Note that this feature will require you to have some free storage in iCloud, as that service comes with a fee that scales based on the amount of storage you choose.