@dalias @hipsterelectron the de facto definition of a reply is any post that sets a value for the inReplyTo field, and the set of instances that can see it is any instance that received it, plus the few that might later request it.
Whether it gets added to a replies collection is basically never a factor.
It's also relevant that there is no way to query a collection. All you can do is iterate it. So the only way to confirm an objects presence or absence in a collection is to iterate the whole thing. Which contributes to the wild west authorization.