Not sure what they used before, but in my younger days when I used to buy cars that were at least a decade old or older, I don't remember the rust ever being as bad as it is on cars now.

My last new car had a corrosion warranty and the manual specifically stated not to use any underbody coatings (never had it on my used cars), and within two years the heat shields all corroded out around their mounts and the license plate bracket rusted and snapped. After another year, an external pump corroded to the point it wouldn't hold pressure. Had the same pump replaced twice and those both rusted out after 3 years. Wheels corroded, sub-floor panels all rusted, lower door rusted.. all within a decade of daily driving and regular underbody car washes.