I find several topics in the community requesting other features that would really be solved, in large part, by simply allowing a student to view their complete work history. Every question, every quiz, every video, every exercise, for every course.
It seems clear that Khan keeps the data because it's required to track mastery, though it's conceivable that you could track mastery and toss out the data from which you assessed the mastery. But, assuming that the data is already there, all that's needed is to build a web page to display the data.
I started a course this week that, when I started, showed I had completed a bunch of psuedo-random parts of the course. It's possible I have checked some of those topics and forgotten but I certainly don't remember doing exercises, etc. So what I wanted to find was where can I find every lesson, every quiz, every exercise, that I have done, including start and finish times - or at least finish since that's a clearly identifiable event.
Other requests that could be solved with this information are requests to drop retakes on quizzes. I understand that retaking until the score is 100% might mask problems with the topic but having the history available for review allows mentors to identify potential skills gaps without discouraging students from studying and moving forward successfully on their own. I'm confident that there are other requests in other community threads where this detail of history would be helpful.