Criminals will always find a way. Make a surveillance state, and they'll just break the law and use encrypted communication anyway. Might even hide data in other data if necessary.

That said, I'd wager that there are quite a few of those communities hidden in plain and unencrypted sight (discord, fediverse, etc.), but they just keep it small enough to not be found (The ones on discord did get found out eventually, but probably just moved platform). So the question would aris: why do these exist when we apparently have the resources to monitor EVERYONE given the chance?

Best you can do is to report communities and places where it runs rampant to the relevant authorities. That's much more efficient than the authorities having to make privacy-violating laws and crawl the net themselves.