Two of the main new features found within Apple's original beta release of iOS 11.4 were AirPlay 2 and Messages in iCloud, both of which were announced nearly a year ago at WWDC 2017 but so far have failed to be included within any iOS 11 public release.
Two weeks after the first beta releases, Apple has released its second developer betas of iOS 11.4, tvOS 11.4, and watchOS 4.3.1. Messages will prompt users to turn on Messages in iCloud on first launch after upgrading to iOS 11.4 beta.
iOS 11.4 and tvOS 11.4 once more include support for AirPlay 2, Apple's upcoming wireless streaming protocol that supports features like multi-room audio. For example, messages deleted on iPhone automatically disappear on an iPad.
With Messages in iCloud, users can see all of the conversations synced between iOS devices and their Mac using iCloud. AirPlay 2 has also resurfaced and Apple is introducing a new ClassKit framework in iOS 11.4 for developers. It would seem that this round of updates is meant to wrap up all of Apple's promises for iOS 11 and its counterparts.