Who gets to define “mostly”? Every public Internet forum censors, whether they say they do or not. Even the *chan sites filter out and delete spam, griefing, child porn, etc. They have to or they would be overwhelmed and useless. The spammers have been advocating for maximalist free speech forever.

I think the compelled speech argument is extremely weak, we have very narrowly defined and specific cases in strictly defined circumstances. This would make it the default for vast swathes of common communications and as I pointed out introduce huge problems with noise, spam and griefing.