Just guessed another two undocumented endpoints "deadcomments" and "deadstories", these take you to a special admin login page if you're logged out, or say "Unknown." if logged in:
/flagged, /upvoted, and /favorites, with an "?id=<your_userID>" are how your own flagged, upvoted, and favourited posts/comments are displayed. "&kind=comments" specifically returns flagged comments, otherwise posts. For upvotes and faves, the argument is "&comments=t", for consistency one presumes...
Favourites can be displayed for other users, upvoted and flagged cannot (for mere mortals).
I suspect downvotes can also be displayed, though I don't know that syntax.
https://news.ycombinator.com/deadcomments
https://news.ycombinator.com/deadstories
And the endpoint "flagged", which is empty for me:
https://news.ycombinator.com/flagged
But if you add a username, e.g. "?id=dang", it says "Can't display that." instead:
https://news.ycombinator.com/flagged?id=dang
Interesting to stumble upon these even if they do nothing for a non-admin user!