So good. I had to write my first review

Kuba, this is the most amazing idea (it deserves more than 5 starts) Unofficially I give you 10 (**********). I don’t understand how WordPress didn’t add a simple notes field like this before. I’m using a Theme that allows me to use different editors, that is crazy, but it helps as some editors allow elements that other editors don’t implement, and it was a mess keeping track of which page was done with wich editor. Now, I just add this to the Note. Well done.
FYI. WordPress says that this plugin hasn’t been tested on the latest WP version (5.8.1). So far it is working as intended.