Although Nostr was not specifically designed for private notes, its relay model is most suitable for them. Additionally, compared to public notes, private notes face less censorship because they are encrypted. Private notes require less from Nostr's design. 

 are you in touch with @JeffG on his latest? i need to catch up to learn more, but it’s a nostr private messaging project aiming at fixing some problems that the current ecosystem of nostr DMs has 

 I know he is writing a nip about signal protocol.