dozens of sometimes-overlapping communities assume that everyone else is operating on the same wavelength

Yeah, that's also a thing, *for sure*. One of the most salient tweets I saw on this whole situation was from catvalente, who pointed out that SFF writers are "workers without a workplace", and a lot of what's happening now is an attempt to use Twitter as the HR department (full tweet here). It's a little different for fans who aren't pros, but the analogy still holds to some extent.

My best guess is that we're going to see more fandom activities taking place in closed communities with explicit codes of conduct and the ability to ban members.

I totally can see that, and even understand why it's been happening, but it makes me a little sad, too.