I don’t feel something like crowdsourced tagging would work for the Fediverse unless it has some sort of approval system built for either post’s author or commuity admins (and those are already busy). Otherwise it would be far too easy to do things like brigading, or pushing government-style “self-censorship” (or straight out censorship of others: just get an army of bots “volunteers” into one instance, let the resulting blocks federate). Something closer to AO3 style tagging, where the author retains most control but readers can add tags to things that are valid only to them (and maybe to people they share data with too?) should workbetter IMO.

Blocking keywords is not reliable to block topics because a keyword does not a topic make, for example in this post I mention queer, socialism, musk and islam yet it’s not topical to any of those things.