Old games don’t use “HDR” the same way we use it today. In old games, enabling HDR makes the lighting calculations in the game engine have infinite range which will then be mapped onto SDR colorspace, which is all software and very much supported in Linux.
If anything the screenshots show a gamma calibration issue. From my experience on Linux native Team Fortress 2, the in-game gamma slider does not do anything.
I don’t know about other people, but the only thing I don’t like about Signal is that it is centralized. It seems to be the only option to actually get everything right for security though from what I hear.