If a user creates a new activity and wants all their followers to see it except 1, how can this be implemented? Do we simply push the activity, and then immediately delete it from the specific follower's timeline feed? This seems like a hack.
this use case hasn't come up before. Why would someone want everyone except one person to see a post? Do they want that person to unfollow them? Are there "rings" or levels of people to choose from when posting? If that's the case, you can create separate feeds with follows to them for those levels (and will likely need to use the TO field as well since fanout only goes 1 level deep).
There's no built in mechanism to specify which feeds to fan out to or which not to. The fanout is intended to happen as fast as possible (milliseconds) so doing those kinds of checks wouldn't be optimal. Your solution to quickly delete from that feed will work.