it really isn't outside Tumbler and Twitter vocal minorities pretending it is. It just is a synonym for "stupid", "idiotic", "foolish", etc especially here in Canada, my friend's home country of Australia, and its neighboring New Zealand. The only time I've ever seen anyone consider it is a slur is in the US.
I really dislike ppl trying to make this into a slur as well. I'm both neurodivergent and 2SLGBTQIA+ and so is my lil sis and we both have a cousin who is austistic who we love (he's only going on 8 though). All of us use retarded all the time just as we and our friends who are gay use fag, queer, and gay to self identity with all three actually having been slurs for decades, yet you'd never see a gay person getting offended being called gay or tell another off for self identifying as queer, gay, or a fag.
I call myself "fucking retarded" all the time since that's how I feel in that moment. Is it suddenly not okay for me, as someone who is both gay and neurodivergent, to use so called "slurs" to refer to myself? Are stupid, idiot, dumb, fool, mentally impaired, autistic, etc going to become "slurs" too? We'll have no words to not only refer to ourselves but for neurotypicals to use to refer to situations such as the medical field where someone needs to know if you are mentally impaired in some way to give you proper care in treatment.
Would you expect my autistic cousin who is severely scared of strangers to be treated like a neurotypical since we're not allowed to call him autistic due to it being used a slur so often? How about me who is nonverbal and types to people on my phone irl along with some korean and québec sign language to communicate with friends, families, doctors, and strangers on the rare occurance I talk to a stranger. How am I supposed to let them know I can not speak, but can hear just fine if they immediately assume I'm simply deaf and I have no words to express I'm not due to every possible word I can use to explain having been or is still actively used as a slur?