I'd say Obsidian (just over five years old, since its first release), which is ironic because it's basically just a UI on top of text files.
Markdown also falls outside the pre-2000 window as well. But, it's closely based on email and news conventions.
In theory, it's significant better than org-mode, because Electron has much more abilities than Emacs. In reality, it's a matter of taste and personal requirements. Obsidian is customizable, so you make it do whatever you want, and there are many addons available; but org-mode has also a very specific focus on the type of addons being available and builtin stuff it has, were Obsidian is more lacking I would say.
> Obsidian isn't open source, isn't plain text enough, and is slow.
It's very fast for what it offers. And "plain text enough" is again a matter of taste. It's all plaintext, but delivering a useful and very powerful interface on top of it. The kind of area where Emacs is lacking.
There's little you can't do with Emacs given it's a small C core running a Lisp interpreter and both the Lisp code that make up Emacs features and the compiled core are open source.