I’m asking for private notes, that only you as a user can see.
Just to exemplify, a similar feature is over at mastodon, where you click on profile and you see an “add note” message, another implementation is over at discord.
It is a really useful feature when needing to remember certain users, like when you are looking at why you blocked someone or for when you interact with certain recurring users.
Is there an extension or feature for this, if not, has it been requested on GitHub, or am I restricted to having to keep a manual list myself?
Among other things, that kind of info is private enough that the server should not store it or even see it. You chose an innocuous example “Poster X is from Belarus” but it might have easily been a private thought that you don’t want seen by others. You have to assume anything on the server will eventually escape. So, that info should only exist client side.