@Gargron The closest thing I can think of right now is discord servers where anyone can join but you only get access to the "welcome channel" until you've introduced yourself.. Maybe write the bio and send a message to mods with the questions they've asked..
To me it sound mostly like a filter to avoid spammers and then it's down to "did you at least read the question". or it might be to keep kids out if your instance is adult focused.. IDK
In a decentralised social media it makes sense to host yourself. That's what we decided to do. This instance is run by two nerds, mostly for the why not of it. feel free to join, and we'll hit you up with an "Hi, who are you?".