The point is, it's not your website if some moderator somewhere can search-replace your content, probably without you knowing.
>>> use pejoratives like "retard"
Retard should not be a pejorative. It is a medical term.
It's fairly well-documented[0], and the GitHub incident in particular has been discussed on HN at length[1]. I think I gave the exclusion that if you're talking about it in a context that's socially acceptable (e.g. critiquing the use of the word, or as you point out discussing it medically), then I imagine that's fine (but I don't know, and I'm not about to test GitHub on how readily they'd censor content in a gray area).
This point about whether a site is yours or not is a red herring. For the purposes of technicality, you could argue that you don't really own your online presence unless you own the servers and the service providing access (per example: Wikileaks used Amazon's S3 service until AWS dropped them[2]). For the purposes of this discussion, however, it suffices to say that you "own" your online presence if your front page (absent a personal site, your profile pages on social networking sites) is determined by you rather than potentially influenced by friends (e.g. tagging you in a photo at a party).
[0]: https://en.wikipedia.org/wiki/Retard_(pejorative)
Admittedly, it's not clear whether GitHub would take down such a repository/site, but my hunch is that when it's deliberate and not casually thrown around, people wouldn't have as much footing to complain about a charged word. That being said, I'm not willing to test it, which says your point has merit just by virtue of a chilling effect.
Nevertheless, a sibling comment mentions using S3, which (I think) would be less likely to get taken down (although I've never heard of Amazon outright refusing to take down such content, so maybe the request simply hasn't happened yet).