Exactly what channels? The police? A lot of what RMS has done isn't illegal and generally there's a very high legal bar for harassment.
So people eventually share their stories through social media and even that doesn't work sometimes (see: Riot). His behavior doesn't have to cross into illegality to be reprehensible.
For a start just leaving a paper trail of formal complaints at MIT would help ascertain this has been happenning for a long time. I just find it shocking that there is no formal complaint. Instead, what we have is just random people claiming that other people got harassed. None of this is verifiable and honestly if internet has thought us anything it is that everybody lies in social media just to get a few likes.
You realize that that very well could be the case and reason why Stallman stepped down, correct?
MIT could likely get into deep trouble if they chose to publish all of the formal complaints against Stallman for obvious privacy reasons and this incident could've just been the proverbial final straw.
And regardless: Apply Occam's Razor here. What is more likely: A group of random people consisting of professors and students bringing grievances against Stallman, or some sort of conspiracy where all of those people are lying, embellishing etc for some form of revenge? If multiple disconnected people are calling someone an asshole, maybe there's an issue here. It's amazing to what lengths people will attempt to create a narrative defending RMS.
> A group of random people consisting of professors and students
A group of random people with a certain radical ideology, most of which work at google[0][1][2], the military[3], or have personal issues with him due to their history[4][5], etc.
I certainly do not believe that it is a conspiracy - ie they are being honest on the fact that they do not like him (not necessarily on why they do not like him though) and that they are not being paid by someone to do it, but they are far from random people.
Also, I personally saw no professors being involved, although I might be mistaken.
So people eventually share their stories through social media and even that doesn't work sometimes (see: Riot). His behavior doesn't have to cross into illegality to be reprehensible.