I have seen them before but the problem is the valve always leaks and eventually fails.
Better idea is to have a manual cutout welded into place either just after or just before the Cat.
My friends Camaro has one of these installed after his cats. A y-pipe is welded in place with a metal cover over one end held on by three thumb screws. All he does to run open exhaust is to reach under the car and pull the three screws off. This lets him run stock mufflers but use open exhaust on the track. The cutout was worth about 15 hp on a chassis dyno.
1989 LE Bonne 122,000 miles
Custom Open Element filter system, Strut Tower Brace from 2nd gen bonne
Light Up Pontiac symbol in grill, 16 wheels from newer Bonne/G, performance tires, performance ignition system, Upgraded brakes.