First of all, any claiming that Public (government) 3rd spaces are not disappearing is either outright lying or being willfully ignorant.
As we become a lower and lower trust society, Public (government) 3rd spaces are being overtaken by violent crime, drug use, or simply anti-social activities. And instead of fixing the endemic problems, governments are simply cracking down on use of public spaces. Out of respect for new immigrants, we no longer have festivals in public spaces. Schools used to be public spaces, as the playground was a public playground, and as long as you acted in a manner that was appropriate around children, you were accepted. Children would literally just run up to a random stranger for uppies, and the stranger would be judged if he did not comply with the random child's request.
Parks are heavily restricted, and if a one adult sits too near to the child play area, the police will often be called, and they will be excised. People keep trying to push the library as a 3rd space, when the library is for RESEARCH. That's why you have to be quiet. The only community thing that should be happening in a library is a public book reading, either by a local author, or by reading children books appropriate for children, (I loathe that this last part has to be explicitly stated). Instead of using libraries, we used to either use the community hall, or - literally - any other part of the city, for community events. Everyone seems to have forgotten that public libraries also include archives, preserved news papers and magazines, historical documents, that kind of thing.
BUT ABOUT PEOPLE THAT DIDN'T FIT IN WITH MAINSTREAM SOCIETY.
This is - quite literally - Tumblr.
Alts. have always found a place to gather, but more stringent and labyrinthine regulation has made it almost impossible to rent small meeting spaces anymore. Every commercial districts used to have a number of (shady) backrooms you could rent for gatherings. As, people loudly proclaiming that libraries are a 3rd space, always, always forget, CHEAP places also count. Pubs and coffee shops are being regulated out of existence, it's harder and harder to rent a back room somewhere, and if you do, because it has to meet arbitrary government standards, you can't do it cheaply. My boss renovated the old garage, and was forbidden from having PRIVATE gatherings there until it was properly bullshitted by the government, considering he just redid the flooring and one face, it legally should have been exempt from government bullshit oversight.
Without even the tiniest amount of hyperbole, I can relate a story. We're on a well. The old well was shit, he wanted a knew one. He made it bigger than he needed to, and this triggered bullshit government oversight. He spent 8 months trying to get a simple overview of the filtration system, and we only got it because the old well crapped out, and we had to shut down for a day, putting us at the front of the queue.
Any Public (government) 3rd place requires you to satisfy a bullshit panel of karens to make sure your group of alternate weirdos (affectionate) is properly government certified for diversity and inclusion. Which should make everyone fault, the fact that those excluded by society have to active cater to THOSE THAT EXCLUDED THEM in order to find a gathering place.
One of you made a spicy tweet, years ago before the xeetification of twitter, well, your group is banned from all Public (government) 3rd places. If you try to find a private third place to gather, the throng gaggle murder of karens will descend upon it to pile five years of government regulation on it to prevent you from gathering.
You seriously used to be able to rent a (nearly) abandoned building/room on the outskirts of society for dirt cheap, so your group of alternate weirdoes (affectionate) could meet to... do whatever you wouldn't post on your main. Use your imagination. Or just type LGD into Tumblr's search, and a lot of fun/horrifying ideas you never would have thought of will suddenly cross your threshold.
Seriously, Tumblr WAS the place for alts. to meet, and they nearly took that away, and they are trying to take away AO3 as well. I've been skulking around the dark shadows at the periphery of the internet since before the World Wide Web was invented, and you literally needed to know the weirdo's (affectionate) computer existed before you could even engage with them.
And since, karens, communists, and appealing to the (fictitious) larger audience has destroyed nearly every place where people who do not fit in with mainstream culture can gather. The Barbie website used to have forums where girls could safely gather to?..
(I honestly don't know; following female conversations is trying)
It's gone, WWOEC is gone, Hentai H(e)aven had it's hostile takeover (non-business), torrents are more often viruses or don't exist anymore,
Tumblr is only inhabited by the ghosts of those too strongly attached to move on, the old, (good), Battle.net is gone, and the only solace we have left, vtubers, are being treated like second-class citizens.