Hey there,
as an exercise I build the requested plugin. You can find it here: GitHub - gnunicorn/discourse-plugin-profile-notes: add and share notes on profile pages of discourse instances · GitHub
It works sufficient enough but the design needs some more work (I take mockups but also CSS, if you guys have any good ideas). From the README:
This neat little plugin adds the possibility for any user to leave private, personal notes on another persons profile allowing them to read them later. Only the user, who created the note can view them.
On top, Staff can decide to share the note with the rest of staff. In which case other staff will see what that person noted on it. This works on a per-note-bases and doesn’t inhibit any staff person to still have their own personal notes.
Looking forward to hear your feedback.