I've been using this method for 25 years, but ruthlessly delete completed tasks and things I decided I don't want to do after all. Kind of like inbox 0, but for my _todo.txt
I would probably keep my notes if I had to report to anybody or needed to keep a track of what I was doing, but luckily I haven't needed to do that for a long time.
Same, I have one living document that is constantly being updated with TODOs, questions, notes, but once they are done or irrelevant, I delete them. I am actually surprised how many people here use append-only approaches.