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.
I'd definitely agree with you on that one. Also notice how the company doesn't push monthly subscriptions on people and just lets their program exist out there.
Logseq for me. Its just so powerful, the infinite nesting and draggable indents and zooming
But it's not, it's a database. That is annoyingøy hard to move around and version control
I don't think it's better than org-mode, but org-mode is also post-2000 so doesn't count here. Obsidian isn't open source, isn't plain text enough, and is slow.
Markdown also falls outside the pre-2000 window as well. But, it's closely based on email and news conventions.