Automatically appends QSP s=review-count-rank on page start
< Amazon auto sort by review countについてのフィードバック
The QSP does not work in All category mode, and hasn't for years. Pick a category and it indeed does still work.
Forgive me, you're right.
That said, you might want to add that limitation to the script description to avoid confusing users like me who don't really use the category feature :P
Amazon seems to have removed the whole review count rank option, at least in the US. So this no longer works, unfortunately.