There are still lots of advocates of various "rustproofing" processes; I am certainly not one of them.
Rustproofing approaches seem to fit in two categories:
- tarry stuff to protect exposed surfaces
- oily mists sprayed in enclosed areas
The tarry stuff is particularly annoying to me. This is the stuff which plugs drain holes and
causes rust; it is also a major pain in the neck for anyone working on the vehicle later. It was created decades ago when the manufacturers did nothing for the bottom of the vehicle beyond a normal coat of paint; this hasn't been the case for many years. This type of treatment is certainly still sold; some Toyota dealers around here display a car (typically a Yaris) rolled on its side on a stand, with half of the underside covered in tarry stuff, and the other half unsullied. It's an advertisement, but looking at that stuff - or worse, touching it - should drive anyone away.
The sprayed-in mists would make sense if there is exposed bare metal in the areas to be treated... but there isn't. These treatments are regularly repeated, and I suspect that carefully looking at the body and addressing any concerns (unplugging drain holes, fixing scrapes, etc) is the real value of the process. I don't let anyone drill holes in my sheet metal.
I drove a 1984 Tercel for about 17 years and while there was lots of rust by the end, it was generally started from places where rock chips and scrapes had compromised the paint. It was not "rustproofed" aftermarket. Certainly the floor didn't rust out like my parents' 1971 Chev wagon (which dissolved in less than 8 years), but to be fair the Chev was driven in a high-salt-use area, while my Tercel was in Alberta.