I think of using a special key in the recipients to show properties as Public or Public- (= only fetchable by authenticated users) fails being appropriately extendable.
For example as @jenniferplusplus points out in this reply to indexable, one should be able to indicate on a per post basis if a post is indexable.
This would add the next special URI https://indexable.example. Next, maybe I want posts one is not allowed to share, so I add the special URI https://notshareable.example, and so on…
Conclusion: We combined two things, for no obvious benefit, but a clear detriment: Creating another aspect of ActivityPub parsing that is non trivial. As I already consider parsing ActivityPub objects too complicated, I would prefer another approach.