It would be very helpful to see the papers that I recently accessed. In other words, the fact that I clicked on a paper title or opened the PDF is a very strong signal that this paper will be relevant to me in the near future. When I come back to Paperpile tomorrow, I will want to continue browsing the papers that I was browsing today.
One way to implement this feature is to add another option to the "Sort by" menu called "Date Accessed".
This feature request is similar in spirit to existing features in other software, such as the "Recents" folder that automatically shows up in the Finder.app window in macOS. This folder contains all the files that you opened in the last few days.