Its not that they were removed - its that originally they were never installed on 67 Mustangs and in turn Shelby's
It was a safety thing years later, so Ford, Mercury and Lincoln sent out hundreds of thousands to registered owners, applied them to new cars being built and sent boxes of them to dealerships who were instructed to place one on every car on the lot (new and used) and every car that came into service. All of these efforts meant that even low mileage cars and in turn owners received then and placed them on otherwise original cars.
Only Boss 429's with the little finger guards and some other other later cars got them before everyone got them in the early 70's.