Oh, is there a threshold? How many people have to be offended by it before it counts? Can you give me a number? I find 'idiot' to be a derogatory slur, does that mean nothing to you?
And note what I said in a previous commend about the elder meaning of 'idiot.' Pretty much means the same thing as 'retard' but people don't think of it that way anymore. You're taking such a traditional mindset about this, kind of "just what your parents told you."
You don't have to agree with a criteria to be able to apply it objectively; it just has to be something that can be evaluated by anyone to come up with the same answer. You can't objectively evaluate whether art looks good; you can only get subjective answers and attempt to find consensus. You can objectively determine if a word is a noun, verb, adjective, etc (given context, as some words can fall in several categories). And given context, you can objectively determine if a word is a slur. (Personally, I avoid both slurs and insults, but github only bans the former, not the latter, and that seems like a reasonable position to take.)
So, to be clear: there's an objective criteria for "what is a slur"; there's no objective criteria for "what is acceptable" or "what is offensive". But in any case, a hosting company is hardly required to provide an objective criteria for what they accept; it's nice if the rules they supply can be objectively evaluated, but that doesn't mean their choice of which rules to apply needs to be objective.
In any case, I have no further interest in arguing with you; I doubt you're going to stop being outraged that someone won't give you a free platform for hatefulness.
Slurs are considered a problem because they're offensive. Banning them is the first step on the road to banning offensive content.
Who arbitrates what is offensive? Sure, we can agree on "The N-Word", but how about Idiot's Guide to Web Tools?