it’s because I used a trainer while playing Left 4 Dead in single-player mode. I only did this for fun in Survival mode, and I didn’t realize I could still get banned. I thought that as long as I used it in single-player, it would be fine.

As proof, here is a link to my discussion about it:
https://steamcommunity.com/discussions/forum/9/3014556944198044348/

I later learned that you need to use the -insecure launch parameter when using a trainer in L4D2, because the game still considers you online even if you select single-player mode.

I accept the ban and understand that it was my mistake.