You have to be careful in the way you use nigger or retard on HN. Calling other people a those will probably result in a comment killed by user flags. Repeated use will result in a shadow ban. Casual use of the word retard (not in the context of mechanisms) might see a few downvotes to poke people into more creative insults.

Most people don't want to use such words and are glad they aren't used in normal conversation. They don't miss cross-burnings; they don't miss similarly violent words.

This is probably a cultural thing. I live in the UK which does not have a free press and which does not have freedom of speech. Most UK citizens find the US position to be extreme. Most UK citizens don't worry about banning Westboro Baptist Church from protesting funerals.

(And, again, this is assuming that Github has actually done something. They might not have; they might be ignoring all reports about the retard word and this is all a troll who faked an email.)