Internet rumor, NRA may be partially responsible for its existence, I think at one point they made mention of some politician(s) or organization calling for it. Who doesn't love a good conspiracy story though.
Consider though, first you have to make it decay predictably in a manner that makes it inert, not unstable, or in anyway more dangerous, which requires it to be compete and sudden, a gradual decay could/would lead to bore obstructions, I have faith this could be done, at a price. Then you have to make absolutely certain it never somehow finds its way into the military or law enforcement supply chain, military probably not so difficult, LE potentially very difficult. Then you have to brace yourself for potential lawsuits from users, or the kin of, who found themselves dead or serverely injured after the ammo failed to work in defensive events. You would also have to get it from the assembly line to the dc, out to the stores, and on to the individual users within a year, outside the current hysteria ammo does not normally move that fast in a lot of markets, dcs and retailers won't eat the cost of hundreds to thousands of lots of ammunition gone bad. I believe the industry would fight this tooth and nail, and you would hear about it, keeping it secret would be a very bad idea, in fact I suspect each cartridge would likely bear a expiration date and the boxes would be plastered in so many warnings you'd have to search for pertinent brand and type info.
