A bunch of proposals for variants of doing that stuff have been (and continnue to be) suggested. I don't know what is the best way (they all look flawed to me given that there is no single source of truth for what is the latest update on the subkeys of each person) -- but even with all that decided and solved I think it's a bad idea because it would add so much complexity to clients and/or relays that Nostr would essentially cease to work or instantly centralize around a single big entity that would become the central source of all truth and the almighty index of all events. That's why I'm considering these other ways.