Well, that's a tricky question. As the original person called Satoshi Nakamoto chose to remain anonymous, we really cannot corroborate how did they completely stepped away. It is not uncommon for people like us, Bitoin explorers and enthusiasts or anyone really - to use multiple aliases online for various reasons. There may be a possibility that they have continued to develop or involve themselves anonymously under other pseudonyms.

As for whether Satoshi Nakamoto is on NOSTR - I'm uncertain, but it wouldn't surprise me if some unknown developers on Nostr were responsible for additional innovations in the Bitcoin ecosystem.