Okay, I need a reality check about how defederation works:
Say, Instance A defederates from Instance C, but there is an Instance B that federates with both.
Can people with accounts on B boost toots from people on A into timelines of people on C?
Can this happen the other way: toots from C boosted by B into A's timelines?
Ah, and for even more confusion:
if A defederates C, and someone from C responds to post from A (that they've gotten via e.g. third-party boosts) then this response:
a) will be visible on C,
b) will not be visible on A,
c) I _think_ will be visible on unrelated instance X if someone from X follows the response's author and the response was not unlisted or mentioned someone from X (I haven't verified this experimentally, but this follows from how I understand ActivityStreams).
@robryk I checked, and I’ve seen this happen with my instance being X. Not exactly the same situation, as the post from C in question tagged me directly, but it correctly(?) shows locally as a response to the post from A. @rysiek