Runs out of the box
use magnifier
The text is quite small. I could only read the text when holding the Steam Deck up in front of my face. I couldn't read the text when I placed the Steam Deck in my lap. However, when I started the game Steam gave me a helpful warning about small text and reminded me that I could press "Steam+L1" to use a magnifying glass.
Works perfectly fine.
No issues encountered

5.9-GE-7-ST GloriousEggroll
Proton-5.9-GE-7-ST

Proton-5.8-GE-2-MF GloriousEggroll
First of all I had no idea what to do just based off other people posting here beyond googling for Media Foundation DLL. So I'll try to explain it a little more in detail...
Go to https://github.com/GloriousEggroll/proton-ge-custom/releases and download Proton-5.8-GE-2-MF.tar.gz (the newest 5.9 didn't work so it has to be this version.) After you have downloaded that, extract it. Then copy & paste the "Proton-5.8-GE-2-MF" directory to "~/.steam/root/compatibilitytools.d/" (if compatibilitytools.d directory doesn't exist, then create it! I had to do create it...)
After that, restart steam. Go into your steam library and right click WARSAW > Properties and enable "Force the use of a specific Steam Play compatibility tool" and select "Proton-5.8-GE-2-MF" and that's it! Now enjoy playing WARSAW!
Proton-5.8-GE-2-MF
The game needs the mf trick. See https://github.com/Matoking/protontricks/wiki/Custom-scripts
Media Foundation is required, otherwise it gets stuck on a black screen.

Game requires Media Foundation to be installed. Appears to work flawlessly once MF installed.
Requires DX11. Hangs on black screen after opening logos; likely an error to load a movie.
Does not load with Esync disabled.