Every "scientific" name for mental disability eventually becomes a slur or name. Idiot and moron were considered proper terms at one point in time. "Retard" was never proper but is easily derived from "mental retardation", which was. In the 80s/90s there was a push to use "special" as a euphemism and it was immediately picked up as a slur, I think both usages have been long-since abandoned as a result.

Autistic is also being used this way but its long-term fate is not so clear to me.

In general euphemisms cannot keep up with bigotry, I rather consider it a lost cause.