Warhammer 40,000: Boltgun was updated by Auroch Digital recently, and now it should be properly fully playable on Steam Deck and desktop Linux. Note: Focus Entertainment previously sent me a key.
Even though Valve listed it as Unsupported for Steam Deck, as reported before it does work and you can get it to perform well enough to be a really fun time. However, the videos didn't play. With an update released July 6th, the developer has adjusted it so the videos now play with just Proton 8 and no other changes. Testing on desktop Linux (Fedora KDE) and the videos work just fine there too now.
Performance also seems to be on the whole a lot smoother, hitting far closer to 60FPS more of the time now.
So if you like your boomer shooters, and especially the Warhammer setting, this seems like an ideal time to go and pick it up. You're in for a good time if you do.
You can see my previous video on Boltgun below for more footage:
Nice to see more games getting improvements for Proton and Steam Deck.