Before Reddit, Facebook, and other massively centralized forum hosting, the thousands of independent, individual forums and discussion boards didn't seem to have too much of a spam/bot problem. Just too much diversity, too much work to get accounts on thousands of different platforms to spew your sewage.
"Sign in with Google" and "Sign in with Facebook" was the beginning of the end.
I'm sure a LLM would have no problem creating an account on all 1000 if someone cared enough to try. Sign in with google is the easy way, but it wouldn't be hard to do sign up for each individually.
Some of them are doing that, but they are either not getting many members (not always a bad thing), or they accept everyone who can act human (which a LLM can do close enough). Sometimes there is a probation period, but it wouldn't be hard for LLMs to write enough to seem real.
"Sign in with Google" and "Sign in with Facebook" was the beginning of the end.