Core Audio added support for FLAC in High Sierra, macOS 10.13, in 2017.
Apple has supported "ALAC" for longer, which unlike FLAC uses only integer math and is therefore less power hungry on mobile devices. You can transcode losslessly between FLAC and ALAC.
I can't comment on the "power hungry" part, but FLAC only requires and (to my knowledge) has ever only required integer math. Source: just looked at my own FLAC implementation [1].
The FLAC decoder has been almost completely integer for a long time (and may now be completely so). Apple Music and iOS still do not play FLAC, which is one significant reason why I won't buy an iPhone. Transcoding my entire music library certainly isn't "just works".
iOS supports FLAC, you can play it in Safari and the Files app. It's true you can't add it to the Music app, but you can use another app.
Also, the $9 Apple headphone dongle is objectively superior to most audiophile DACs, which isn't surprising since it has a larger R&D budget than that entire industry. (Same goes for Google's IIRC.)
I would guess these days, many people buy audiophile DACs for the amplifier stage, for use with demanding high impedance headphones. However, there have been some really bad phone DACs in the past, the Qualcomm audio from the Note4 that I used to own sounded significantly worse than the iPhone using IEMs. What are people measuring with regard to the Apple dongle? Is it "objectively superior" with regard to multiple measurements?
It shouldn't be too hard to build a decent DAC and amp stage, at least for ear buds and IEMs, it's just that many manufacturers (e.g. Samsung / Qualcomm) don't care. Apple sell music and so probably should care more.
It's still useful to have an amp for very high impedance headphones for electrical reasons, otherwise it can be quiet and lack bass (plus drain the battery faster.)
On macOS AVFoundation (and so QuickLook and QuickTime) can open .flac files. The Music app can only playback a artificially limited number of formats and can't play FLAC.
Apple has supported "ALAC" for longer, which unlike FLAC uses only integer math and is therefore less power hungry on mobile devices. You can transcode losslessly between FLAC and ALAC.