Open-source and unmoderated are 2 very different things.
Lemmy is open source so you got this already.
As for an instance without an admin, there can’t be such a thing. An instance has to be hosted on some server and this server belongs to somebody that has to maintain it.
A community without a mod doesn’t exist, as a community has to be founded by someone to exist.
Now, do you really want a community without an active mod ? Well if you want spam, bots and shitty content, sure.
If your problem is power tripping mods, just leave the communities with such mods.
Or you can run your own Lemmy instance and see how it goes.
People have tried the “anarchist forum” approach and it literally never ends well and devolves into chaos. Even 4chan has ground rules and moderators (known as janitors)
I know a lot of people have a knee-jerk reaction to hate them but the platform is really much better with them.
Roll your own? The thing is most of us left Reddit because of decisions from ownership. Administration forcing Reddit apps to pay insanely high fees while not fixing the official Reddit app to be more accessible for blind people and to have the tools needed to moderate communities is why we left…
The people that left because of “micromanaging moderators and admins” left when they closed r/jailbait if you wanna invite those creeps back go ahead and make your own
You kind of have it right now just go to usenet. Pretty much zero moderation, anybody can say anything, but there’s nobody there to listen. Because it’s too noisy. Heck even the spammers gave up, there was nobody there to spam. It wasn’t even worth spamming anymore
You can always run your own server with your own rules even if that rule is no rules.
The beautiful thing about the fediverse nothing stopping from someone running a server like that. Maybe common sense but nobody else is stopping them.