It's harder to police that word when a lot of us would use it self-referentially.
That's a slur doesn't mean that the slur doesn't also do the best job of encapsulating what the person means. The baggage the slur carries is exactly what makes it the most precise, most easily understood way to say what they're wishing to say.
A lot of people (especially when we're young) don't really understand the difference between intellectual impairment, slow cognitive tempo, developmental delay, learning disabilities and extreme social impairment, especially if the few people they've met who those labels could be applied to have more than one of those. They don't understand the differences particularly so their diagnosis is never more precise than ret*d, and once that label is assigned they assume the entire stereotype because if they could understand different things get conflated together under that label they'd already have the nuance to understand differences between the proper terms.
It's one thing to complain about people calling other people that word, but if someone gets hostile over someone else's choice to use it self-referentially they need to be told where to stick it and how.
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