You're free to use retard in your code as much as you want, apparently not on Github though. They, as a private company, are allowed to pick and choose what they want on their site.
There's actually an entire ranking for those with disabilities - just below a "normal" person was the "feeble-minded", which ranked ahead of a "moron", which was more functional than an "imbecile" which was the last step before an "idiot".
This was all common parlance in the 19th and 20th century.
So explain to me again why retard is specifically offensive.
That is all. Really. Give it a few decades and we wouldn't be having this discussion because no person with mental disabilities would feel offended by anyone using it as a mere synonym for "dunce".
Of course so-called Internet feminists might still demand your head on a silver platter even then, but (in stark contrast with actual feminists) nobody should be paying attention to them to begin with, even if they make it hard not to.
> idiot: MEDICINE (archaic): a mentally handicapped person.
Google "euphemism treadmill" for more info.
[1] http://englishcowpath.blogspot.com/2011/06/euphemism-treadmi...
Just switch back to imbecile for the next 100 years? Problem solved? Clearly not, the underlying issue has nothing to do with the word but meanings that people attach to them and that is something that will not be fixed by outlawing the word as the meanings attached will shift to a new word.