This was central to Tumblr leftist philosophy as recently as 2014. I remember it well. I can't help but feel like this is in no small way down to the rise of TERF-ism.
But also a phenomenon that happens literally everywhere you get humans interacting about anything at all, which is the creep towards the binary. You start with, for example, a very good position like "The bulk of social discussion should be intra-community, because those are the people actually affected." And then that drifts, just a little, into "The opinions of the directly affected people have more worth than anyone else", which... isn't quite the same thing? And then it drifts more and more, until by the end it's "Unless you are directly and incontrovertibly in the affected group, you must entirely be silent and never speak on this topic AT ALL, not even to express a personal preference for an aesthetic e.g. which of the 27 lesbian flag designs you personally like best (genuine discourse I have seen)."
And like, obviously I get it. I've sat through plenty of cis men weighing in on feminist stuff with the most moronic and ignorant takes imaginable. But I actually think my leftist husband, a basically cis-man, has considerably more valuable thoughts on the role of gender and feminism than some tradwife creep who wants us all to return to a world of one baby and zero orgasms per year because she hasn't understood that she could just do BDSM and then she'd still get her own bank account.
I think the main area I'm personally seeing this in on Tumblr atm is in neurodiversity discourse, actually, and in particular as it pertains to autism. We've now gone rapidly from "ND folks are an oppressed group by an NT social majority and we should discuss all those avenues of social nuance to push for equity" to "ND folks are unique and perfect unicorns who always without exception get along perfectly with each other and are never ever at fault for anything or capable of being assholes, and literally all asshole behaviour on the planet is done by those evil mean NTs who shouldn't be allowed to raise children anymore. Also if anyone ever dislikes someone ND it's because they're an evil ableist sea witch (all genuine discourse I have seen)."
And again! I get it!! God do I get it. When you have been made to feel ashamed your whole life of who you are, Christ there is so much healing in instead being proud. In particular, in taking aspects of yourself that others have denigrated, and saying no; these are objectively good, actually. And it is tempting, isn't it? To take that extra step? To say these are BETTER, actually.
And then before you know it you have posts circling with thousands of notes claiming that if ND folks live together they all magically do each other's accommodations perfectly without a single clash but not a single NT has ever successfully provided accommodations in their lives so we should segregate the population (no, bro, the eugenics is good this time I swear), and anyone who disagrees gets the "Boooooo you're an evil ableist who hates autistic people" treatment that OP talks about here.
It's the same phenomenon you get with terminology too. People learn a new term for a thing, and then it drifts as it creeps to the binary. Gaslight, Karen, trigger, sustainability, cancel, literally, queerbait, special interest. Nuance is lost as the heavy cudgel of easy meaning comes down. And, I think, it's also tied in with the urge to form a rigid and highly parochial community. We see this particularly in the queer community, of course - for a social movement that's supposed to be about removing the boxes, so many queer people are absolutely obsessed with making new boxes and then gatekeeping/enforcing them as strongly as cishet society ever did. It's a stupid and mean part of human nature - we're a social species, so when feeling threatened, we want the support of our tribe around us, but that means needing to define exactly who the tribe is ("Queers only! But not those icky trans people, and aces are basically straight somehow, and bis are dangerous greedy posers trying to look like us and prey on us..."). Once you've got your tribe, then it's easy to identify threats ("Straight people! And those gross other queers, who might make us look bad in front of the straight people, who might then hurt us!")
And then THAT means that anything at all that shakes that perception of the world is itself a threat. "If queer people bully a straight person for being straight - a thing that absolutely does happen - that is harmful and unacceptable." "HOW DARE YOU SAY THE STRAIGHTS ARE OPPRESSED BY THE GAYS. I CAN'T BELIEVE YOU'RE SAYING THIS ONE PERSON BEING BULLIED IS THE SAME AS SYSTEMIC HOMOPHOBIA. I CAN'T BELIEVE YOU HATE GAY PEOPLE. HOW CAN I, A QUEER PERSON, POSSIBLY BE HARMFUL TO A STRAIGHT PERSON. I'M GOING TO DOXX YOU FOR SAYING THAT."
(Genuine discourse I have seen. Including the doxxing.)
Anyway. Sorry OP, I've been thinking about this for a while, so it was your post that got the addition!