People are constantly uploading mods to the Workshop. There's also players coding Sourcemod plugins that add lots of new functionalities to dedicated servers.

Too little compared to l4d2.

Yeah well, the fanbase is way smaller. The game already has a ♥♥♥♥♥♥♥ of community content via custom maps. Learning to complete them on both Classic and Nightmare already provides many hours of replayability. You can also manually modify many game settings to create a custom difficulty to suit your tastes.