The first beta of iOS 11.4 brought back Messages in iCloud which was removed from the public release of iOS 11.3. AirPlay 2 has also resurfaced and Apple is introducing a new ClassKit framework in iOS 11.4 for developers.
New in iOS 11.4
● Messages in iCloud
iOS 11.4 beta includes Messages in iCloud for testing and evaluation purposes. Messages will prompt users to turn on Messages in iCloud on first launch after upgrading to iOS 11.4 beta. Please note that Messages in iCloud is automatically enabled for users who have two-factor authentication and iCloud Backup enabled.
Developers can download the new build from the link below. Please follow iClarified on Twitter, Facebook, Google+, or RSS for updates on what's new.