I spoke to Cal for this article though I don't think he used much of our interview. I developed GTD software back in the first as Kinkless when became OmniFocus after I joined Omni for a year.

Today I don't use any "super specialized" tooling for task management. Intentionally. I don't like being wedded to any given app. My tools are Apple Reminders (universal for my family since we're all on Apple devices) and Obsidian (or really just plain text / markdown, accessed currently through obsidian).

Lots of thoughts about all this but in short there were some good ideas I took from GTD (universal capture being the biggest, but that's not really a GTD unique idea) but most of it I've jettisoned.

(my obsidian / markdown usage is basically "take notes, sometimes notes become projects, those projects automatically show up in a dashboard" and mixing notes, content, and tasks organically)