When in doubt check out Protondb, it's a database that says whether games work on Proton or not and people often put fixes there.

Left 4 dead's page is here: https://www.protondb.com/app/500

It seems like it should work but you need to right click the game --> properties then add -w 1920 -h 1080 as launch options.

When it happens I usually run my steam from the terminal, some errors from the game would appear there

That's strange it works flawlessly on my SteamDeck. I can check what proton version it's using later today.

Try cycling through other proton versions as well. I don't really understand why some games will work on older versions and not newer ones, but it happens.

Any particular reason why you are using Proton? I believe there's a native version of it for Linux (or at least there used to be).