From GameWatcher: “Left 4 Dead was a bit of a zeitgeist back in its heyday. Not only was everybody playing it, but everybody was also modding it, and so a legend in its own right was born. Though it was a great game on its own, it would be hard to argue that it was mods that made it as popular as it was on PC. Earthfall might be going down a similar path, hopefully.”
Source: N4G PC Earthfall Modding? Holospark says it "could consider it in the future"