Org Mode keeps being pushed again and again, but honestly this title is one of the boldest claims I've seen about it. Just look at the document. It's a 400+kb HTML page of Emacs configurations, keybindings and other stuff that makes absolutely no sense unless you've been an avid Emacs user for years. Where is the Plain Text here? I see nothing of the sort. Sure, the output might resemble Plain Text, but it's clearly…
That being said, there are many tools built on the org mode format to help with things like todo management and interactive execution of code blocks, but it doesn't make it any less plain text.