It's not strictly privacy-focused but The Goblin's Notebook is designed exactly for your use-case. It has markdown, object connections, every object has a player visible setting, so your players can access known content while you keep secrets hidden. There's a free tier, a mid tier at $1.50 and an unlimited tier at $3 dollars a month (managed via their patreon).

org-mode/org-roam-ui in Emacs with PGP support synchronization via git in forgejo

I use Docbase, which apparently has been archived on GitHub.

https://opensource.appbase.io/Docbase/

https://github.com/appbaseio/Docbase

crypt.ee

I haven't tried yet (because I'm not a very organized person) but pushbullet.md

I had this exact problem a whole ago, trying to find an alternative to OneNote. I went through many of the other suggestions in this thread but settled on Trilium.

Super easy to setup in a docker container. Self hosted so I control my data and access. Can by accessed via reverse proxy when I'm out and about.

Notes can be a mixture of text, pictures, code (with formatting based on language)

They are arranged in a directory structure with notes inside other notes under chosen topics down the left, and open notes are in tabs along the top, much like One Note.

The chrome extension allows me to quickly snip and send back screenshots or Urls of sites I'm on, and the android app let's me make quick notes which are filed away by date for later organisation (when I get round to it...)

The only thing it doesnt support, that I wish it did, is multiple users. I don't see why you couldn't just make another container for each user, just not very practical if you have a lot of users.

I'm using Trilium notes. it's simple enough and does what i need. Used to use Obsidian but wanted something open source, and with Trilium you can self-host the sync server for free (even comes with a handy web-ui).

Note that it is much simpler than obsidian, but for me it's plenty. It was easy to import my obsidian vault into it, and it allows exporting as .md files which work fine back in obsidian too.

Recently the dev said he's putting it into maintenance mode, so no new features will come to Trilium. There's a community around Trilium Next that wants to keep expanding it, but personally i hope Trilium stays as it is and is maintained for a long time.