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?
admins can access the data, the data is only encrypted between your computer and the instance server. the system is not necessarily designed with keeping your content secure more than just your account. this is the case on most social systems however, DMs are not truly private, nor are private rooms, the only time you can have some level of assurance of actual private communications is with end to end encryption
matrix, another fediverse network has you covered for if/when you really care about that.