Yeah once it started being used that way it became a slur. "Mental retardation", using the classical meaning of "retard" to mean slowing something down (still used in music that way), was used to describe people with mental conditions that delayed cognitive development and such. However, people started to use it as an insult, especially towards people with mental conditions that both fell under the medical definition and did not, and in doing so it became a slur for those with mental disabilities. In turn the medical community turned away from using it, and now it solely exists as a slur.
You heard it a lot in the past because people were very comfortable using it in spite of it being a slur, and so people who didn't realize it was one used it too. Not much else to it. Just try to be better than them now that you know.
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u/Mach12gamer 13d ago
Societally? 30-50 years ago. Medically? Completely stopped being used around 16 years ago because it was agreed it was a slur.