My open source analogy wasn't great, but the point I was trying to make is that even things we usually think of as open are compatible with consent. Similarly we're used to thinking of federation as unconstrained (well except for Gab) (and everybody else who gets blocked) but that's just the specific flavor of federation that's been practiced on the fediverse so far -federation's compatible with consent, at least in my books.
Power-hungry instance owners can already decide not to federate with other instances, arbitrarily or for any reason -- counter.social's an example. Consent-based federation just changes the default. It's true that this changes the equation a bit; today there's a small amount of effort required not to federate, a consent-based approach flips that and there's a small amount of effort required to federate. At the end of the day, though, power-hungry instance owners are gonna do what power-hungry instance owners are gonna do; threads.net and mastodon.social are going to make their own decisions about federation policies no matter what the free fediverses decide.
On Lemmy? Certainly not. But on other fediverse software, there are followers-only posts, direct messages, local-only posts ... none of it's encrypted, but still it's not public.