I found existing posts asking "can I make Table of Contents always display?", with the answer "no you can't." I couldn't find a way to easily add clear "+1" for "I really want this feature", so I'm making a post.
(I guess consider the actionable question here more like "I'd like it to be easier to register disagreement/unhappiness when a Paper Question's answer is 'it's supposed to be this way'")
I get the value of distraction-free writing. But I use Paper as a tool to conduct meetings with my team. The documents quickly get quite long, and I'd consider it very important to know that other teammates can see when I've added another section header, so that they know they can contribute to that portion of the discussion. (Whereas if there is no Visible Table of Contents they're more likely to duplicate discussion in different parts of the doc)