Check out this post, having travelled to a distant node in the !fediverse without being directly addressed (only indirectly, through being a reply to something that originally had the attention of a user on that instance) to a user of a remote instance - despite not being subscribed to from anyone there!

https://quitter.es/notice/885881

This is however _not_ "parent retrieval", but I think it's an important step (and should've worked since quite a while actually) for more complete conversations.