MR has literally never been common parlance or slang. The words "mental" and "retard" have, but outside of the medical community nobody was using it and especially not as a handy insult, because it isn't handy. It is a clumsy and unwieldy way to say it. No kids have ever run around calling mentally ill people "MR", this is a completely false narrative and the expectation that nobody would look into it is typical of someone fabricating a lie.
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