If you're cool with Nazis and CSAM, sure.

Given the nature of internet commentators: yes it is possible but it won't be topic related for long and will devolve into pure anarchy and Godwin's law will be invokable in less than a day.

The more people in a space, whether that's online or IRL, the more important moderation/regulation becomes

It's one of the big reasons why urban city-dwellers and rural people often have very different politics

That's called 4chan. You can make a version of 4chan on the fediverse but good luck keeping your hosting providers happy when the content stored becomes more and more vile (just read their wikipedia page to find out their troubles with that)

Spam is as old as the internet. I think a public service with zero moderation is going to have spam without moderation.

If you like nazi bars with constant arguments and nastiness, sure.

Sure. Briefly.

You mean, is it legally possible? Technically possible? Or are you imagining what would happen if there were no moderators?

There are many public forums that are not actively moderated, in that the administrator responsible for moderating can't be bothered.

Technically, yes, at least insofar as the elimination of a specific 'moderator' position. You could find ways to distribute the duty to users or something, but you still need something to control content or you will end up with something posted which brings government level antagonism.

Only with some heavy restrictions.

Imagine a forum where you can, by default, only see the messages of people in your allow list.

That way spammers and people with bad intentions will just scream into the void.

Anonymous forum?
Or only selected membership?

That sunny episode is a good example of how it’ll just get exploited

You'll have another 4chan that way.

Craigslist?

Yes everyone just abuses the people who are acting up until they stop posting. It works great.