It's not impossible but it will take a good chunck of the user's data bandwidth to solve in a native nostr way. Also, there is no clear definitiom to what a follow is. The contact list is just a contact list. It doesn't necessarily mean people are following the contact list. And now with lists and other algorithmic feeds, the "follows" number is starting to make less and less sense.
Which nostr clients are notifying that somebody follow/unfollowed you? I see primal.net webapp does this, but it is the first one I noticed. Amethyst certainly doesn't. Does damus or anyother in ios? nostr:npub1gcxzte5zlkncx26j68ez60fzkvtkm9e0vrwdcvsjakxf9mu9qewqlfnj5z nostr:npub1xtscya34g58tk0z605fvr788k263gsu6cy9x0mhnm87echrgufzsevkk5s nostr:npub16c0nh3dnadzqpm76uctf5hqhe2lny344zsmpm6feee9p5rdxaa9q586nvr
Discussion
I assume primal.net is doing it using their backend caching layer which aggregates data from all known public relays and offers a more wide view of the numbers and the possibility to notify when something changes. I wonder what this caching strategy means more generally for nostr as the protocol. And if it will be widespread used by other clients. nostr:npub16c0nh3dnadzqpm76uctf5hqhe2lny344zsmpm6feee9p5rdxaa9q586nvr