I'm certainly not campaigning for more use of the terms "retard" and "retarded". On the other hand, I've experienced first hand the very common, reflexive and thoughtless use of these words. Whenever it's used in my presence I make a point to express my distaste for the term and I encourage others to stop using it. I've gone through these motions with countless co-workers and two people higher up on the management chain, including my direct supervisor. But where I live, it's practically a regional colliquialism.
In this case, perhaps removal or correction is reasonable and Github is acting responsibly; I've already established that I have problems with these terms and do find them offensive. Still, I am uncomfortable with the whole episode and I don't feel at all confident that the next time this happens, it will seem as reasonable. On the contrary, I expect that the next time this happens, it will end up being more about ideology as it seems more and more people conflate "I am offended" with "I disagree".