They're still available as an aftermarket accessory. Usually aimed at off road 4WD enthusiasts, like me. I have one and it works very well indeed. My diesel engine doesn't mind a bit, and the bag has a valve to stop it deflating when you remove the hose from the exhaust pipe.
Need to be aware of protrusions under your vehicle that may puncture the bag, especially if you've done any under tray mods like extra batteries, storage racks, water tanks etc.