Obsidian, featured in the meme, is on Linux, and for a while now free of charge (but with paid official plugins for syncing and web publishing), and I’d say is pretty good.

I honestly just take notes in sublime text myself, which is paid, but I’m paying for it anyways and often have it open in the background. It can store unsaved tabs in some session file, so I can just put unimportant notes in a tab and have them available to open later.

I also want to mention nextcloud notes, I don’t use it for much so I don’t know how good it is, but I have a nextcloud instance and use the notes for simple notes I want to be able to access from my phone.