Urban legend.
The lineage of this thing goes back to the "expiring primer" myth that went around during the Clinton administration.
Myth: Ammunition companies, in cahoots with the Federal Government/New World Order/Space Zombies, were rumored to be making all primers in such a way that they would have a shelf-life of a few months.
Truth: Winchester shipped a batch of primers (Winclean, IIRC) that had a few duds. Somebody ran with the whole silly idea.
Scary thing is, I'd forgotten about the whole thing until about three months ago, when I was confronted with a panic-stricken guy who wanted to be sure the primers he was getting were not the "new" kind that would expire
The Internet: everything old is new again.