All you need to do is run these 2 commands.

sv_master_legacy_mode 0 - appear in steam server browser
sv_master_legacy_mode 1 - appear in game server browser

You can automatically run these commands by add this to the end of the server.cfg

wait 1
sv_master_legacy_mode 0
wait 1
sv_master_legacy_mode 1

Thank you Sghirate for showing these commands


For players joining the server, it's recommended to turn down the texture detail setting to avoid crashing mid game. The game may still crash when changing levels though.