On text Varnishes the misses list shows this URL:
as an outlier, with receiving approx 1700 req/min right now. For reference, the second on that list (also an outlier, probably a bug) is receiving 70 req/min.
From the headers, I see that it's all Cache-Control: private, expires immediately.
I have no data on when this began and it could be old, but I don't remember seeing it before. From the git logs I see commit I06ee0f01d896bc66545a1800b24693ce7524e433 (ChangesListSpecialPage: Separate all functionality for generating feeds) that changed some feed internals and URLs, so perhaps this is related?