Publicly, that is. In a private forum like Hacker News, exclusion of disgusting hateful speech is legitimate and even necessary for the survival of the community - or people would leave because the "trolls" have taken over.
Private forums like Hacker News in no way infringe on the rights of others, by using their own property (web site) in a way they deem fit.
I'm not sure that barring disgusting speech is "necessary," if only because "necessary" can be such a loaded word.
reddit, for example, seems to have a principle of free speech, where there only a very specific things you are not allowed to say board-wide. So there is a fair amount of disgusting speech there. It seems to work for them -- although I have been slowly disengaging from it over the past year or two, and maybe that's part of the reason.
The sitewide bans, to my recollection, are of only two things (besides that which is illegal): sexualized pictures of children, and doxxing.