An example is a private user adding the content item to their library. We may also not route these kinds of activities to the content item, but having a "content activity" feed kind of invites that kind of feature in the future.

Things like "messages" feed, and if we had a "previews" feed, they wouldn't invite those kinds of activities and would probably be safe to let non-members/anonymous access, but an "activity" feed is a little different and should be protected a bit better as we do with group "activity" feed for the same reason.