Censorship, beyond what’s required by law, is doomed to fail.
So you can lose credibility two ways, one by not doing any censorship because people on the internet will be the worst if you let them. Doing too much censorship is also bad because people don't like that either. Of the big causes of censorship currently, I think of things like youtubes copyright claim process and how that is routinely used as a censorship backdoor by anyone - including the police. Sometimes its not even for any good reason and done by unthinking bots. This is banning more perfectly fine content than anything the government has done. I don't understand why there isn't more pushback against that process to punish people for frivolous claims.
If you look at every attempt to create "The Uncensored Free Speech Version Of [ANY_SERVICE]," they all, inevitably turned into a 4chan-like trashfire. You've got to have some kind of moderation.
So whats changed?
Well, I have my thoughts, but one thing is for sure, as soon as the platform itself tried to start moderating, that's when things really started changing.
This statement is hysterically ahistorical. Each thing you listed had active moderation. Sysops didn't just allow anything on a board, if you posted stuff off topic or offensive (to the Sysop) it was removed with prejudice. IRC networks all had long lists of k-lines of people kicked off the network. Individual channels had their own mods with ban lists. Forums either moderated or were deluged with spam. AOL and ICQ were both highly moderated.
Just like today small networks might be uncensored free-for-alls. Even then they are/were rarely actually uncensored, it's just you might have not been censored because you aligned with the views of the owners/operators.
The only really uncensored free-for-all was Usenet and that state only existed for its first decade or so when it was limited to professionals and academics. The Eternal September turned Usenet into an unusable mess. It's corpse limps along today as a vehicle for piracy and not much else.