Long ago, my father would inspect hospitals and would have to call in his reports and I’d frequently hear the r-word - in its original form - in those instances, at least originally. And like you say, that word was originally meant as a euphemism since it just means “slowed”.

A term that was more frequently slung around on our playground, probably more than the r-word was “sped”, which took me a long time to connect with the shorthand for saying SPecial EDucation.