> That's supposing law enforcement has viable ways to leverage knowledge of its existence into investigation and prosecution.
Which they do, because that's their job and they do it all day long.
We're somehow talking about both end to end encryption and moderated public forums, but those are two different things. If you have a public forum where anybody on the forum can read the messages then anybody on the forum can read the messages -- including law enforcement. So they join the forum and start investigating in all the usual ways, and get a warrant to have the ISP upstream from the site start logging its traffic so they can start locating the users and getting warrants to bug their homes etc.
When you have end to end encrypted communications, the only people who have the message are the sender and the receiver. Then there is no dedicated CreditCardSkimmersChat site, they can use any generic secure messaging software for that. But this isn't any more difficult for law enforcement than criminals who communicate in person -- you still have to come by some reasonable suspicion of them to begin with somehow, and once you have you get a warrant to install bugs, which overcomes end to end encryption because then you're collecting data at one of the endpoints.