In my particular case, I just didn’t know it was enabled (my modding guide mentioned a way to stop it, but I guess I did it incorrectly). The game hadn’t received updates in half a decade, and I don’t really use Steam for anything else. Apparently, I wasn’t the only one in that boat.
Most tutorials I can find involve enabling the steam cli, then using steamdb to look up the “depots” of previous versions and downloading the old update in chunks, then unpacking and copying the old game files to your install location. Not exactly convenient.