Forums had all the same rules and rule enforcements for moderation. (Agree to the community rules, agree to the community terms of service, verify your email account, for the first few hours/days of your account you see only a curated subsection of the forums, etc.) It's a lot of the complexity in forum software that accumulated over time because it wasn't handled by "third party bots" and instead had to be baked into the forum app itself, which lead in part to why forum software is so hard to build/run/host because for a good, well moderated forum you need all of those moderation tools and generally can't just bolt them on with a "Forum Bot" from someone who isn't the forum software's author.

This has already happened. That forum is Reddit, for quite a large proportion of niche hobbies where someone would come in and ask a question.

And it has the same problems of one-way escalating moderator strictness.