https://wccftech.com/apple-starts-seeding-the-first-betas-of-ios-11-4-1-macos-10-13-6-watchos-4-3-2-and-tvos-11-4-1/

Apple started seeding the first beta of its upcoming macOS High Sierra 10.13.6 update to developers for testing purposes, while macOS 10.13.5, an update that’s expected to include Messages in iCloud, is still in the testing phase as we speak. If you are a developer, then you can download the macOS High Sierra 10.13.6 beta through the Software Update mechanism in the Mac App Store as long as you have the proper profile installed with you.

Side by side, the company also started seeing the first beta of the upcoming iOS 11.4.1 update to developers. This comes a day after releasing iOS 11.4, which comes with an addition called AirPlay 2 as well as support for Messages in iCloud.