I would like to search for “not done” notes and file a feature request
You already have a great distinction with “on agenda” and “done/not done”. I don’t like the idea fiddling around with tags suggested here. It is unnecessary.
The way someone (like me) could use it is as follows: I put different notes of topics in. Some of them I am currently working on - so I set them “on agenda”. Others I use to either work frequently on long-term topics or they have a lower priority and I work them when I have time. The notes “on agenda” are easily found. One click. But currently there seems to be no way to filter for open Notes.