In my case the location is in '~/.steam/debian-installation/steamapps/common/Left 4 Dead 2/left4dead2/cfg'

After you have went to that directory you need to add a new line to video.txt & videodefaults.txt

Add

"setting.mat_tonemapping_occlusion_use_stencil" "1"

to the two files (make sure that the spaces are the same)

Make video.txt & videodefaults.txt read-only
I'm not 100% sure it's necessary to make it read-only but I have it read-only currently, I'm gonna mess around with it.

Right click your game and add "-autoconfig" to your launch options

And that's all!