I got hooked on outliners in the prehistoric days of the "Trash-80"; a 32k RAM/ 32k ROM notebook. One fabulous upgrade was a ROM chip, made by Travelling Software, called SuperROM. It had a wordprocessor (like wordstar) a spreadsheet (like Lotus 123) and an outline processor (like nothing else) called Thought.
It was brilliant ! easy to use, and would support links to text files . I even found an ancient freeware DOS outliner that would read the files (once uploaded to the 286, via LapDOS)

I've been ruined for linear files generation, ever since !

that DOS Dinosaur was PC Outline, from BrownBag Software