Yes, it can be sprayed liberally over the bottom of the car to protect things and will even help keep things like parking brake mechanisms better lubricated over the long run.
Waxoyl or something very similar was sprayed on the outer areas of my TR4's frame, right at the factory. I think that was done to protect during shipping. Just a few years ago I finally cleaned the last bits of it off around the parking brake mechanisms and upper side of the diff housing. That was the first those parts had been off the car from new, so I am certain the waxy gunk was "original". It was quite thick, up to 3/4" in some hidden recesses, and had done it's job quite well for 40 years... no rust underneath it. But, it only stuck around in hidden and somewhat protected areas. Any place that was exposed to tire splash, car washes, etc., any traces of the gunk were gone long before I got the car in the 70s.
Back when I worked at a car dealership in the 1970s, something similar was being used on many new cars, paticularly imports but also on some Detroit iron, and we did a lot of cleaning to remove it before a car could be delivered to a customer. The underside of the car and hidden recesses were never really cleaned, though.
I would feel okay about coating the suspension and underside of the car. I wouldn't use it on the exterior or under the hood, though. It ain't pretty and will melt from heat. It's best in hidden areas, such as inside frames.
In really wet and harsh climates, it's recommended to respray every few years.
Oh, and it's usually sprayed *over* whatever paint or finish was already on there. Being a semi-transparent coating, the color underneath shows through a bit. Being waxy and oily, not many paints could ever be sprayed on over it.
BTW, a friend cut up a scrap TR4 frame and was surprised to find it was actually completely painted semi-gloss black on the inside, same as the outside. Apparently it got at least one coat sprayed on before being spot welded together. Of course, the spot welding damages the paint in the welds' immediate vicinity, and there is no way to respray inside, but most of the metal still had a nice coat of paint after 35-40 years. Perhaps the later cars didn't get that paint inside, and that's one reason the IRS frames seem more prone to rust. Just a guess... I don't have one handy to cut up and check.