https://www.gamingonlinux.com/2023/12/hdr-support-for-kde-plasma-6-seems-to-be-shaping-up-nicely/
While it's far from finished, and still needs some manual effort to get properly working, HDR support for KDE Plasma 6 has made great progress.
Just switched to the beta on Arch and its definitely coming along. SDR content still looks washed out, but its a lot better on my monitor (LG 27UK600) than on windows.
For me I have to change SDR Color Intensity to 100% for the colors to be close to SDR. Although because you can't use ICC profiles with HDR and my monitor is wide gamut the colors are wrong. But as far as I can tell it looks very close, if not identical to running SDR without my ICC profile enabled.
The display I bought a few years ago doesn't support HDR because I didn't think it'd be supported so soon. The Steam Deck and the money put into developing the Linux desktop was really unexpected to me.