Apple added support for FLAC in the new iPhone X, iPhone 8 and 8 Plus, Apple TV 4K, and the iPhone 7 and 7 Plus. It is reported by iClarified.
FLAC or Free Lossless Audio Codec audio format similar to MP3, but Lossless support. This means that audio is compressed, but the recording quality is not degraded.
New format support is specified in the technical specs of the new Apple devices and will launch on an iPhone 7 with iOS 11. However, it is doubtful that Apple will use Music to FLAC, so to play music in this format will have to use the app “Files” or a third-party option.