All Source games, not just Deadlock, did and still somewhat do have a strong modding scene. The way it's handled now is that official Valve-hosted servers have a config option called sv_pure set that prevents most mod .vpk files from loading on connecting clients (though some things like custom HuDs are whitelisted by this system). Once Deadlock gets paid cosmetics you can expect sv_pure to be turned on for Valve servers, which 99% of the playerbase will exclusively play on.