If you like first person shooters, and if you like 90's era games, and you like Warhammer 40,000, then do have a look at this game: Boltgun (steam link) You play as a Space Marine of the Ultramarines Chapter, fighting against traitors, demons, and chaos space marines. The weapons available are your expected and iconic 40k weapons like the chainsword, boltgun, plasma gun, heavy bolter and so on. They even got the grenades right, frag and krak grenades. The developer clearly knows and loves the 40k universe. I have played it for a little over 4 hours by now and I absolutely love it, especially the fact that you can taunt in this game. The graphics are pixelated, and you can increase the level of pixelation in game settings.
Forgot to mention that it runs great in Proton on Steam. There is just one little issue, the few in-game cinematics do not play. You can play those cinamtics yourself from the game directory in Steam.
Wow, sorry I missed this post! I still do not get email notifications from the forums and we have never figured it out. Probably Gmail marking the notifications as spam. I'll definitely check this game out and hopefully there is co-op cause that is my favorite. It kinda looks like Doom meets Valheim graphics in a Warhammer world.