Given that the admin of any instance with a single approved follower can see the contents of the community, this idea feels like placebo privacy. The false sense of privacy could be counterproductive.

The only way I can think to federate with something resembling true privacy would be to use PGP or similar. Encrypt the data with the user's private key, send it to and store it on remote instances encrypted ,and decrypted in JS on the user's computer. That would require users to mange private keys which they would no doubt lose, and be a lot of work for a pretty niche feature.

Awesome to see!

Can't help but think that there's gotta be a relatively straight forward to hack this over ActivityPub though? I'm not over whatever security there is in the protocol, so that likely does not mean much at all.

But how hard would it be for a server to convince a normal lemmy instance that it is doing all the right things in terms of following/subscribing to a private community when it's actually displaying it publicly?

For that reason I wouldn't be surprised if some would prefer, for the sake of caution, to run a private community in local only mode too. Not that a federated private community isn't useful ... it totally is, even if there is a risk.

Giving the idea a fair shot, what did people use private communities for on Reddit?

We had one for testing automod, but either that doesn't really need to be private or you could use a closed instance on Lemmy for it.

Other than that, I know of the meme communities that you'd randomly be added to

A late pattern in Reddit was personal subreddits - communities named after the account that created them. They were infrequently used, but it provided a smoother pipeline for people who lurked or commented in existing communities to become comfortable making posts and moderating communities themselves.

Ideally these communities would be prevented from appearing in the "Trending Communities" list or local/global feeds unless someone other than the owner was subscribed to them, but wouldn't be private in the sense that no-one could see them. Just they wouldn't get wide distribution.

Another pattern is the "Country Club" post - where individual posts in a community could be limited to people verified to post in restricted threads. This comes from BlackPeopleTwitter. The individual verification method is likely not the only way to achieve this. People who comment or vote could be limited to only those who share the instance, are subscribed to the community before the post is made, or are members of instances whitelisted by the community.

Both of these patterns are interpretations of 'private' to mean 'restricted' and not 'secret'.

I don't really see the point, but it seems to be something people want.