Apple has quietly included support for playback of FLAC audio files on the 4K Apple TV, iPhone 8, and iPhone X, with compatibility also added retroactively to the iPhone 7 when iOS 11 gets its public release.
Support for the lossless compression codec now appears in the technical specifications on Apple's website for all of the above devices. However, as it stands, Apple's Music app does not currently support the format, so device owners who want to listen to the higher quality audio files will have to use the native Files app or a third-party app specifically made for FLAC playback, such as VLC or Plex.
Apple's support for the FLAC codec doesn't officially extend to the iPhone 6s or iPhone SE, which may be because an A10 processor is a minimum requirement for hardware decoding and Apple is not satisfied with the power consumption tradeoffs of FLAC software decoding.
That said, Apple's latest iPad Pro range should also be capable of FLAC playback thanks to their high performance A10X Fusion chips, yet Apple hasn't updated its iPad Pro tech specs to indicate they will also support the standard.
It's possible that Apple is still testing FLAC support for its range of mobile devices, and may even have plans to offer the lossless compression standard as a download option in iTunes and playback in the Music app further down the line. However, it's worth noting that mobile users would only get the full benefits of FLAC by listening using Lightning-connected wired headphones, since a Bluetooth audio connection doesn't offer enough bandwidth to make the experience worthwhile.