I did a FEP about posting things to places other than your timeline/feed. Initially for walls, and Smithereen implements it for walls right now, but could be useful for much more.
For some implementations, like mine, it’s technically challenging to have all actor’s activities in outboxes. For example, I store likes in a separate DB table from posts, and I’d have to merge that all somehow and be able to paginate through it. Only for the client to probably split it back apart.