I mostly do this, but with a physical notebook. I wouldn’t be able to work with a .txt file because it’s too limiting: you can’t draw anything, you can’t easily make arrows between stuff, you don’t have the nice mental reward when you strike some item off your list.

This is for the day-to-day organization. For the rest, I dump all my knowledge in a wiki (MediaWiki), and I use iOS/macOS’ Reminders app to remember things to do far in the future (like "cancel XYZ subscription" in 6 months) or at very specific times.