I assume clients subscribe to kind 3 with their own pubkey as tag. So they won't get that note, as the pubkey is no longer in the list. Clients need to either listen to all or requery all kind 3 events, or periodically check the kind 3 events of who they think are following them.
Discussion
Ah you're asking about how a user's client detects that someone has unfollowed them. Yeah if you look at how Damus does it, it re-queries all relays a user is connected to to get an up to date list of who is following them.