All I know one has to do a really complex command line mpv invocation for it to work. Dunno about other apps. Firefox definitely doesn't work, but that applies to Windows as well as far as I know.
I use Linux daily but honestly I don't game on it and I watch movies on my proper hdr10+ dolby display so HDR on Linux was never even a thought in my head.
There are people who have problems with HDR, mostly people with TVs that have no business trying to display HDR content but the manufacturer added the feature anyway because sales.
If you have the right TV, the right streaming box, and the right receiver (if you use one), HDR looks fine, but a lot of TVs either don't support HDR or have a really terrible HDR implementation. And then a lot of streaming services either have bad non-HDR video streams or just take the TV claiming HDR at face value and don't give you a way to turn it off.
I don't think it's fair to expect everyone to extensively research and buy high-end TVs just so they can (literally) see a movie or show.
My monitor states it does HDR but the nits are like only the bare minimum for it to be labelled as having HDR that it's not worth using. My tv on the other hand supports hdr10+ and Dolby vision so actually does look worthwhile.
None of my displays support this kind of HDR so some games (mostly those with no way of turning it off) end up darker than they would if I had a display that supported it. Haven't had other media do this though.