How to subscribe to a thread?
Several days ago FEP-efda: Followable objects was published. I don't like this solution because ActivityPub spec only talks about "following" in the context of actors, and the proposed "proxy-following" mechanism forces us to change some well-established practices.
So here is an alternative: FEP-f06f: Object observers.
Object observer is an actor that can be followed to receive object updates. If conversation thread is a collection, its observer will broadcast Add and Remove activities that have thread collection as their target. Observer's followers will have an up-to-date view of the thread.