Interesting idea - from reading the Zerobin project page (which, incidentally, is unmaintained) they do client side encryption and the server stores the (encrpyted) message against an ID. The client then tacks the symetric AES key onto the share URL which you then send to the other recipient (and doesn’t touch the server). It’s a simple method of creating a secure channel, and relies on a single shared key known only to the client(s).
That would absolutely work in a one directional situation where a creator is sending a note to several clients. But if said clients also want to modify the note, things would get complex fast - how would you maintain sync between multiple clients who may be editing slightly different versions?
You could solve this by descoping collaborative editing (which in fairness I put into scope about a paragraph ago by inferrance). The recipients would get a read only copy at the point of sharing, but that would be it.