I actually know the history of the word retarded as applied to children with developmental delays. Around 1960 clinicians started using the word to label children with delays. And it took about 3 years for children to start using it as an insult.

The bad thing is what these people are doing isn't depriving the English language of a slur. They're actually turning words into a slurs. And depriving the language of words with perfectly innocuous meanings.