Apple has released iOS 11.4.
iOS 11.4 is an audio-focused update, introducing support for multi-room audio through a new protocol that supports multi-room audio on all AirPlay 2 enabled devices.
The iOS 11.4 update also introduces Messages in iCloud, a feature that has been in the works for several months and was first promised as an iOS 11 feature in June of 2017. Messages in iCloud is designed to store your iMessages in iCloud rather than on each individual device, allowing for improved syncing capabilities.
watchOS 4.3.1 and tvOS 11.4 have also been released, and the software on the HomePod has been updated as well.