I would never use the word retard as an insult to someone with disabilities. I would use the word to describe things I find stupid. Even if I did find the word retard offensive, I wouldn't like what github did here.

Yes, they have the right to remove words they don't like. Yes, I have the right to not use their platform.

Github's time as a central source for code is limited anyway. If you don't like what they do contribute to projects that make the web more decentralized.