Nation matters. In my country by the 1980's mental health professionals were no longer using phrases like "retard" anymore because they were recognised as being slurs.

Source: My mother was a mental health nurse in the 1980's. None of her colleagues ever used that word because by then in my country it was recognised that "retard" and "mental retardation" were very bad phrases and no one should be using them.

I don't think you deserve the downvotes, because you were just asking questions (and actually just asking questions, not in the "I'm just asking questions" way that some shit heads do in order to say prejudiced things without being accountable for them).

But it's worth pointing out that the word "retard" was always used as a way of saying that people with these conditions are less than people without them. It was always used as a dehumanising phrase- it's just that society moved on and people realised that this was not acceptible. It's not that the word one day became offensive, it's that society decided it wasn't ok to dehumanise disabled people anymore.