macOS High Sierra 10.13.5 beta brought back Messages in iCloud for testing. Apple also notes the availability of a 64-bit testing mode that developers can use to software for 64-bit compatibility.
Known issues with Messages include:
• Messages may be unresponsive on launch.
• All conversations may be missing.
• Conversations may be slow to appear.
macOS 10.13.5 beta supports:
• MacBook (Late 2009 or newer)
• MacBook Air (Late 2010 or newer)
• MacBook Pro (Mid 2010 or newer)
• Mac mini (Mid 2010 or newer)
• iMac (Late 2010 or newer)
• iMac Pro (2017)
• Mac Pro (Mid 2010 or newer)
Installing macOS 10.13.5
● Back up your Mac.
Always back up your data and files before installing beta versions of macOS. If you have multiple Mac computers, we strongly recommend installing on a secondary system. Backing up files on a Mac is easy with Time Machine, the built-in backup utility.
● Enroll Your Mac in the Apple Developer Program
Download the macOS Developer Beta Access Utility from developer.apple.com and run the installer. Follow the onscreen instructions to enroll your Mac. To check if your Mac is enrolled in the Apple Beta Software Program, open the App Store pane in System Preferences for the Mac App Store. If your Mac is enrolled, the message “Your computer is set to receive beta software update” is shown.
● Install the macOS Developer Beta
After completing enrollment, install the developer beta from Updates in the Mac App Store. If there is not a developer beta available, you will receive a notification from the Mac App Store when it is released.
Developers can download the macOS Developer Beta Access Utility from the link below. Please follow iClarified on Twitter, Facebook, Google+, or RSS for updates.