Checkpoint64 has a preset for Left 4 Dead 2: it already knows where the config folder is, backs it up automatically and keeps every version. Everything this site has on Left 4 Dead 2:
Left 4 Dead 2
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Config file location
Windows:
C:\Program Files (x86)\Steam\steamapps\common\Left 4 Dead 2\left4dead2\cfg -
Backup guide
Competitive games keep your progress on their servers — what's on your disk is the config. Checkpoint64 versions the settings folder for 67 games and 16 dedicated servers, so a reinstall or a file verification can't cost you your binds. Free download.
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Launch options
-novid-console-window -noborder-vulkan