https://techcrunch.com/2018/05/29/apple-releases-ios-11-4-with-support-for-messages-in-icloud-airplay-2-and-more/

Apple this afternoon will officially release the latest version of its iOS software for your iPhone and iPad, iOS 11.4, which at last adds support for Messages in iCloud, along with other new features, including most notably, AirPlay 2 and an update that allows two HomePod speakers to work together as a stereo pair.

Messages in iCloud was first announced a year ago at WWDC 2017 as a way of keeping conversations up-to-date across all your Apple devices, including iPhone, iPad, Apple Watch and Mac. Its introduction means you’ll now be able to access your entire Messages history when you set up a new Apple device, and, when you delete a message from one device, that change syncs to all your devices.