Where do all the research paper annotations go?
I was recently introduced to this site: hypothes.is (I have no affiliation). In essence, any PDF with a URL when opened with their layer allows you to see comments and highlights from anyone else who has chosen to use it, or sub-groups you can join or arrange.
I have not seen anyone in academia use it, but perhaps you could start. It does seem like it could be useful for a small group of people who want to understand a paper.