"sharedInbox endpoints SHOULD also be publicly readable OrderedCollection objects containing objects addressed to the Public special collection..."
https://www.w3.org/TR/activitypub/#sharedInbox
I disagree with this SHOULD, would downgrade to a MAY.
Minor, mainly because consider a small instance, whose users do not show their Following collections, this could basically leak who the follows are.
That said, I haven't found any Software that does have a publicly readable sharedInbox!