Previewed at WWDC, launching in the fall.
Apple Seeds Fifth Beta of macOS High Sierra 10.13.5 to Developers [Update: Public Beta Available]
The new macOS High Sierra 10.13.5 beta can be downloaded through Apple Developer Center or the Software Update mechanism in the Mac App Store with the proper profile installed.
macOS High Sierra 10.13.5 introduces support for Messages on iCloud, a feature that was previously present in macOS High Sierra 10.13.4 betas before being pulled ahead of the release of the update. Messages on iCloud is also available in iOS 11.4.
The Messages on iCloud feature is designed to store your iMessages in iCloud for improved syncing. Right now, incoming iMessages are sent to all devices where you're signed into your Apple ID, but it's not true cloud-based syncing because your old messages don't show up on new devices nor does deleting a message remove it from all of your devices, both features enabled through Messages on iCloud.
Messages on iCloud also allows your older iMessages and attachments to be stored in iCloud rather than on your iPhone, iPad or Mac, saving valuable storage space.
The update also likely includes bug fixes and performance improvements for issues that weren't addressed in macOS High Sierra 10.13.4, but as Apple does not provide detailed release notes for macOS High Sierra beta updates, we may not know exactly what's included until the new software is provided to the public.
No other major outward-facing changes were found in the first four betas of macOS High Sierra 10.13.5, but we'll update this post should any new features be found in the fifth.
The previous update, macOS High Sierra 10.13.4, brought support for external graphics processors (eGPUs) along with Business Chat in Messages and several other bug fixes and smaller feature improvements.
Update: The fifth macOS High Sierra 10.13.5 beta is also available for public beta testers.