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Forgive me if I'm missing something here, but I can't quite square the original poll options with "of course it should be defined by Bob's settings".
My thought was that and / all settings that Bob would ever have for his own posts should be available to him, and the default should be whatever his default normally is.
Essentially, (my view is) the fact that Bob's post is in reply to something else is beside the point: Bob's post is Bob's post, just like any other he'd make.
I selected other. I feel that Bob's post's visibility should be defined by Bob's settings for that post.
Followers of Bob that don't also follow Alice could see his reply and know that it was a reply to something else, but they'd not see what it was in reply to.
Conceptual parallel: one can comment publicly on copyright protected material that others may not be able to see. one can also comment publicly on classified info (there may be penalties for doing so, but it can be done).
And under "explore" there is a category for "dog" but not for "cat" videos.
About time dogs got their own corner of the internet.