Since Plasma 6. Not sure about HDR wallpapers (never even heard of an "HDR Wallpaper", so I've not looked into support for it), it should work with fullscreen videos though, and Steam games work if you run it through a nested gamescope session with a special HDR Vulkan layer installed.
I assume it will work for a live system? When Fedora 40 eventually comes out I'll give it a try. I can't stand to daily drive plasma unfortunately. (Gnome works better for me)
Maybe if it works well I'll run plasma on a media machine.