It implies that the medical community moved away from these terms because they became insults. However, these are all terms derived from IQ testing and the concept of "mental age". IQ has been de-emphasized as the diagnostic criterion for a couple reasons: people with mental disabilities aren't the same as younger people without mental disabilities; the focus should be on what sort of help people need rather than what they can't do.
> In current medical diagnosis, IQ scores alone are not conclusive for a finding of intellectual disability. Recently adopted diagnostic standards place the major emphasis on the adaptive behavior of each individual, with IQ score just being one factor in diagnosis in addition to adaptive behavior scales, and no category of intellectual disability being defined primarily by IQ scores.
https://en.wikipedia.org/wiki/IQ_classification#Classificati...
As for the common use as an insult, you're right that its impact is culturally relative. To me, I would ask why you're using a term which used to be a medical diagnosis as an insult. The wiki section says kids are saying "what are you, 'special'?" in reference to "special needs". Does that mean it's bad to get that medical diagnosis? I'd side-eye someone who says "retard" as an insult because it's not just rude to the insultee.