Apple has promptly followed the developer betas released yesterday with second iOS 11.4 and tvOS 11.4 public betas. While tvOS 11.4 features AirPlay 2 for testing, iOS 11.4 has both AirPlay 2 and Messages on iCloud for public testers.
The new iOS update seems to have gained stereo support for the HomePod, however, the feature will only be usable with a software that Apple has yet to release for their smart speakers.
AirPlay 2 testing, meanwhile, will require Apple TV 4K or Apple TV 4 with tvOS 11.4 developer or public beta.
iOS 11.4 also includes the new PRODUCT(RED) marketing wallpapers for iPhone 8 and iPhone 8 Plus, and not for iPhone X for some reason.
Note: Using Apple’s developers beta requires paid membership. The public beta program is free for all but involves stability risks, so make sure you have all data backed up before you test it out.