I've removed it here: https://github.com/rust-nostr/nostr/commit/c2442d87da7ec521006a6954bb472f3aef73383e
At the moment, I think there are other things more important to focus on. We'll see if re-add it in the future, after the beta or release stage.
Otherwise, if someone is interested in building a nostr-sdk extension library, could add it there.