There are cases of people who have completed suicide because of bullying based on their LD.
You cannot solve bullying by banning words. Especially in cases when those words are used in an entirely different context, having an entirely different meaning, without any profound malevolence.
The words are malevolent because they contribute to a culture where it is acceptable to treat people with LD as sub-human. You don't get to use a word and deny the long history of that word. This is nothing like, for example, the fake etymology of "nitty gritty", or confusion between the word "niggardly" with "like a nigger". The word retard has a long history and is closely associated with oppression and murder.
White people don't get to say "nigga - no, it's okay, I'm not saying nigger!"
The other confusing thing here is that people are asking for the freedom to do what they like with someone else's property, and when that person asks them to modify their behaviour they lose their shit.
This is just github saying "no shirt, no service". What happened to "their servers, their rules"?