

Don't use Proton, native version is good. Needs tinkering but is easy stupid, and once done it plays flawless.
Delete this file: [game directory]/lib/libstdc++.so.6 and you're good to go!
With that simple tinkering, the game runs just perfect so far. I downloaded the Steam Cloud save from a game started in Windows, and I continued the gameplay for 3-4 hours with no issues at all.
Browse local files for the game and delete lib/libstdc++.so.6
- Download the native version, do not switch do Proton
- sudo steamos-readonly disable
- Copy .so files from gamedir/lib to /usr/lib32 (liblua, libsteam_api, libfmodevent, libfmodex)
- Edit run.sh and change LIB variable to /usr/lib32
- Launch the game, at this point 1/3 of the screen will be black because of some glitch
- Go to settings and disable fullscreen, click apply (button should be barely visible and clickable in right bottom corner)
- Enjoy the native version running on your Deck
Game screen was moved about 1/3 of the screen height towards the bottom, creating a black stripe at the top and cutting off the bottom
Other comments about abysmal performance via Proton are correct, so don't bother with that. Instead, I've used the steps stated in one of the comments to fix the native version, but on Deck it took few more steps.
Well, you gotta choose native(no Proton) as a first step. Then go to properties of the game. Select local files. Click on "Browse" and it will open the game directory in the file manager. Open the lib folder. Copy these 4 files into the root directory /usr/lib32 (I don't even own a deck yet to know if this can be fixed on the deck). On my linux system that directory existed but was empty. When I copied in the 4 files they were the only files there. You don't have to mess with any permissions(phew!), just open a terminal while still in the "lib" folder and sudo cp libfmodevent-4.32.20.so libfmodex-4.32.20.so liblua5.1.so libsteam_api.so /usr/lib32. In the game directory which you got to by clicking "Browse" open the file run.sh in a text editor.
Replace the default line "LIB=$GAMEDIR/lib" with "LIB=/usr/lib32" and save. That's all and it was way too much! I only did this because I didn't want to live the hell I had to live through by trying Proton 7 or any earlier version, the game still was totally unplayable in Proton! Native WON! Thx goes to KM for solving my issue! Game runs as smooth as butter! No weird Proton bugs! So happy it can still work, even with all this totally crazy tinkering!

Wildly unstable framerate at all times
Low framerate makes it unplayable
Very slow, tried a few proton versions

I couldn't get the game to run with the regular linux support. Proton actually worked better than the native support. So good job proton.
Seems a bit laggy

Does not run without modifications on Linux native anymore. You must move the files listed below from the /lib folder in the game director to /usr/lib32 and then change line in run.sh in the main game directory from
LIB=$GAMEDIR/lib
to
LIB=/usr/lib32
Files that need to moved to /usr/lib32:
libfmodex-4.32.20.so libfmodevent-4.32.20.so liblua5.1.so libsteam_api.so