I am not 100% sure, but i would guess that there is no such feature in #nostr itself, rather it’s a decision from the client developer themselves to have a bookmark feature. Therefore it‘s different for every client.
Discussion
Yeah, I get that. But if we’re able to get our following list from client to client I would assume there could be a way to sync bookmarks as well if a client offers it. I’m not a dev so idk 🤷♀️