What's interesting about those is that they were originally medical terms (which I'm guessing @lemmy689@lemmy.sdf.org knows). But, for those who don't:

Moron was coined by a psychologist in 1910. He tried to make it fancy by using an ancient greek word "moros" which meant "dull". It wasn't an insult, it was meant to be a clinical term "used to describe a person with a mental age in adulthood of between 7 and 10 on the Binet scale."

Imbecile "originally referred to people of the second order in a former and discarded classification of intellectual disability, with a mental age of three to seven years and an IQ of 25–50".

Idiot "was formerly a technical term in legal and psychiatric contexts for some kinds of profound intellectual disability where the mental age is two years or less".

Then they became used in popular culture just to insult someone or describe someone who was stupid. Even their order changed. I'd say "Idiot" is the least insulting now, but used to be considered the most developmentally challenged.

In any case, "retard" also started as a clinical term, actually meant to replace idiot, imbecile and moron because they had become too widely used by the public and seen as insulting. Handicapped and disabled replaced retard when it was seen as too insulting. Then things moved on to "differently abled" or "challenged". It's been called the euphemism treadmill.

In the end, saying "retarded" was never illegal, at least in the US. It was just seen as crude. That hasn't changed. It's just that people who worried that being seen as crude might hold them back no longer think they have to worry.