The way Apple handles HDR clearly suggests that you aren't meant to use HDR colors in UIs. The mouse pointer has a pure white (#FFFFFF) outline, but when you put it over an HDR video, that outline appears grey. Then after you've closed the video, your vision needs some time to readjust back to the UI white level. The APIs (Metal and whatever videos are played through, VideoToolbox?) also suggest that it's for games and media content only.
Now, of course you could circumvent this and add retina-burning-bright controls to your app. Your users would probably hate you for doing this.
And regarding wide gamut, I'm not sure what kinds of benefits this brings to UIs either. It does make photos and videos feel more lifelike, especially when combined with HDR. But I've seen an extension to CSS that allows specifying wide-gamut colors, and you can also use them in native UIs (I've managed to do that in my aforementioned experiments https://news.ycombinator.com/item?id=30219169).
> wide-gamut and HDR GUI capabilities that they introduced as far back as Vista
Wow. That was quite the future proofing at the time when everyone was transitioning from CRTs, and LCDs of the time were noticeably crappier than CRTs.
The thing with Windows though is that it's really hard to get so many companies involved in the chain to agree on something like this. Apple has complete end-to-end control of both the hardware and the software/APIs which allows them to casually pull off this kind of stuff.