Hey, I got it today too. On my Ubuntu desktop, it played in Proton 8.04 like butter.
I only played the first 20 minutes or so, I didn't want to spoil too much of the story. Looks really interesting, in any case. No shock, considering this is Croteam.
Here's a tip I learned in another thread on Lemmy today.
If Steam won't let you download the demo on Linux:
Go into Steam Settings > Compatibility, and turn on “Enable Steam Play for all other titles”, Steam will then ask to restart. Install your game (which should now not complain because it’s getting the Windows version for proton) then go change the setting back.