The old stuff contained about 5% nitrobenzene, which gave Hoppe's No. 9 the smell that gun owners loved. The formula was changed in the 1980's when California banned nitrobenzene, because it was a carcinogen. Like many banned substances, small infrequent exposure to Hoppe's No. 9 probably wasn't very dangerous, or most of us would be dead now. But deer and elk had to come to associate that scent with danger, since it carried so well.
The old formula worked better than the newer version because nitrobenzene was a better solvent for nitrocellulose gunpowder residues. Millions of bottles were produced, so the old stuff will continue to turn up at gun shows, flea markets, and garage sales, for years to come.