I'm not on Cachy OS myself but just normal Arch with KDE, and HDR is working fine there at least. Using an RTX 4070. I hear some people can still have issues with it though, but especially when running Wine/Proton in Wayland which is possible nowadays I haven't had any issues regarding that. You still have to use environment variables to enable it though (PROTON_ENABLE_WAYLAND=1 PROTON_ENABLE_HDR=1). Hopefully that will soon just be automatic.
That is for games specifically. For video with mpv it just works in my experience. Browser support is still a quite hit or miss.
That is for games specifically. For video with mpv it just works in my experience. Browser support is still a quite hit or miss.