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)