When I started making games I used a .txt file as a todo list, then I moved to Trello, after that I used Notion and now I'm back to .txt files. I crave simplicity.
@pocket_ @grumpygamer I tend to keep my notes in .md
The difficulty with markdown is that there's a bit of inconsistency between editors and that can be problematic if you intend the .md file for someone other than yourself (or if future-you needs to switch software for some reason or other).
e.g. TeX math support varies wildly between editors.