If I ever sat on a horse that was trained to slow or stop when I used a spur, it would be off my property before you could blink.
I ride and train horses to be well rounded. I want my horses to be able to do a dressage test one day, work cattle the next, jump, and then go and do a working class complete with haunch turns, rollbacks and stops the next.
Training a horse to stop off the spur is the complete opposite of every other discipline, and completely counter-productive if you ever intend to do anything else.
The fact is that the AQHA have made statements saying they want it OUT, and there are other methods to acheive the exact same result, I can't see why anyone would be training spur stops.
I have two horses at home who slam their butt into the ground when I clench my butt and say whoah - one is a less-than-green-broke arab mare. No reins required. It isn't hard, it just takes a little skill, feel and coordination. I can do the exact same stop on my better trained horse without tack.