There's a third layer: the servers. Newsgroups relied on servers propagating messages to other servers as well as delivering them to end users, and they could h

There's a third layer: the servers. Newsgroups relied on servers propagating messages to other servers as well as delivering them to end users, and they could have various types of filters. The same sort of model exists in modern distributed systems like Mastodon/ActivityPub. I can block a user from another server or an entire server individually, or my server admin can block them from communicating with anyone on my server. What's missing that I think will be required for anything that gets popular enough is a means of sharing blocklists automatically, preferably with some machine-readable details so that they're useful even when servers have different rules (e.g. I want to subscribe to bans from Foo if the ban is for hate speech, but not for porn).