1. His post was not an out of context rant on an unrelated school mailing list. He was criticizing the words used by MIT students in connection with a protest at MIT, and that mentioned his ex-colleague. The MIT students posted this in the first instance, and it was a direct response to them. It seems to me like an appropriate forum.
2. Stallman's argument was simply to call a spade a spade, and not a defense of Epstein or Minsky. If you told 10 people (each unaware of any of the facts alleged) that Minsky sexually assaulted a girl, and then asked them to describe what they imagine occurred after hearing he sexually assaulted some, many might assume he violently assaulted someone. The degree of differences in guesses that you might receive makes the word functionally prone to tarnishing someone with a reputation for a different crime than the one they committed.
Stallman therefore asked for the incident to be described in unambiguous terms; for example, that Minsky had sex with a sex trafficked 17-year old 50 years his junior on Epstein's island. That does not mean he defends that scenario -- you can still view it as reprehensible, and condemn it, but at least you are not engaging in "accusation inflation", where you are condemning someone for a potentially worse crime than they committed.
3. I agree that Stallman is unnoticed outside this sphere, but this is part of a continued trend of threatening people's reputations and livelihoods for stating an opinion. I created a throwaway for the first time ever simply for this comment (which I think is relatively benign), so yes, I think this trend will continue to lead to self-censorship.
