The objectionable ingredient...
that was removed from the original #9 formula is nitrobenzene - 'a known carcinogen'.
But, when you take a sniff of the old #9, as compared to the new, your sense of smell says "Yes! That's the real stuff!" (assuming you grew up with the original formula).
FWIW, Bruno's Shooter's Supply was selling the old formula #9 as recently as a few years ago - I bought a gallon. I don't know whether they were obtaining the Hoppe's directly in the old formula, or simply adding the nitrobenzene.
Gun cleaning, as such, lost its charm for me a few decades ago, but having the old #9 makes it less of a chore.
PRD1 - mhb - Mike