That’s weird, Half Life 2 had a Linux version noted in Steam and it runs fine. L4D didn’t have a Linux version noted and it didn’t give me the option to install it until I selected Proton. I don’t know, I’m very new to gaming on Linux so its probably user error.

I own it on Steam and it shows a Windows and MacOS version but not a Linux version so I went this route.

This is what worked. I had to go down to version 4.2-9 and it finally ran.