I think these fragmented Discords are the return to the idea of specific, uncrowded, neatly maintained places, with a relatively high barrier to entry for a random person. Subreddits are a bit similar, but less insular.
I don't know much about Discord (my only experience being some years ago when I joined for an open source project and left soon after I noticed how incredibly use hostile it is) but I do know that if you create a single account it is trivial to join any "server" (which, despite the marketing is just a chatroom hosted on their servers).
The Discords I'm active in are all everyday conversations, like big group chats. Some of them are funny/interesting and occasionally someone gives useful advice, but the vast majority are forgettable.
I think that people should publicly share valuable information (like great conversations or useful advice) and some of their typical conversations (a context summary for outsiders and history). But privacy and ephemeral-ness make people more open. It may be better to have a space for most conversations where they're not expected to be saved, or (because "not expected" in Discord relies on weak evidence and today's norms) guaranteed not to be saved.
My wife and I were recently talking about how we kind of luck boxed into dodging a bullet when we had kids (which was rather late). But it's no wonder so many people had or are having so many issues growing up in a public social media era. It's not only your right, but responsibility, to say, believe, and generally do stupid things as a kid and a young adult. It's an important part of growing up. Nobody should ever have to worry about this period in their life following them around forever.
The point of this sort of thing is that whether it is fake or not doesn't matter. Because it is possible for someone to record a log of your activities, someone claiming they have an incriminating log of your activities will be believed (By a very large number of people).
It might not be believed in a courtroom, but for the other 99.99% of life, we do not apply the same standards for reviewing evidence.
Whether the platform keeps logs isn't important - the platform won't weigh in on this sort of stuff anyways, unless there's a subpoena.
Only if it's public. There are many private Discord servers.
The way they do it is that the default set of permissions is basically none, but then there's a server role which actually gives you permissions to see the channels and post in them. So, anyone can join the server, but only people who have been granted this role (e.g. by admin) can do anything on it, or even see others.