Were most forums pushing the outer boundaries of acceptable speech online? Reddit mods make and enforce the rules. Reddit gets involved mainly when subreddits do borderline illegal stuff. Is this a real problem, or a hypothetical one?

That's a bit like asking "what did most books do?"

The whole point of forums was that it's difficult to make a generalization about them and moreover, what "most" forums did/do doesn't matter. What a particular forum might do in a particular context is what mattered.