Sorry, I made that term up I meant the list of Tags that are underneath the Notebook list. I consider these "Notebooks" since clicking brings up a list of notes, but they aren't as fancy as what you want.
I agree that "Smart Notebooks" that are 'just' saved searches would be really useful, and I would also likely abandon traditional notebooks and use a flat catchall notebook + tags if they existed. (In fact, I've been lazy about sorting my "inbox" notebook lately and it is becoming that already!)
I am wondering if the Favorites saved search would help you? joplin-favorites/README.md at master · benji300/joplin-favorites · GitHub (note you have to copy from clipboard, but fairly automatic)
this has come up a few times, most recently I saw this comment:
It seems like it would be a lot of work, but in my mind it is theoretically possible to do via plugin. Since I am not a plugin dev, easy for me to say!
Not exactly what you want, but I wonder if this in-progress plugin will help (or could be molded to help). Conflict Resolution Plugin
In general you might like this thread if you haven't seen it. https://discourse.joplinapp.org/t/any-suggestions-on-what-plugins-could-be-created/
There are a lot of ideas there. Also it seems like if you write out a detailed description of a plugin you can sometimes get a developer excited to work on it.