OK, went thru my books and found no entries for road salt removal ( or neutralization) from car underbodies and/or painted surfaces. As sodium chloride is an end product for so many chemical reactions it would be hard to break it down. It's such a stable chemical.
I know of removing salt stains from leather shoes (too many Youtube videos) and outdoor boots uses a wipe down with vinegar and (for leather) a deep wash with a mild soap and conditioner. But digging deeper into that I discovered that a by-products of salt and vinegar under certain conditions produces hydrochloric acid. I wouldn't want that residue on my car! Then again it must be a rare occurance, as salt and vinegar goes into salad dressing, so....
I then found a commercial cleaner called Neutro-wash, and looked at it's SDS. Basically sulfamic acid, sodium hydroxide, and isopropyl alcohol. The acid is a relatively safe commercial descaler and cleaner, and the lye is probably to neutralize any excess acidic residue. All commercially available bulk chemicals, so - if you didn't want to buy the name brand product - you could probably make it yourself and use it on the car.
https://rhomar.com/products/neutro-wash/There's this detailing specialist on the Tube (The Detail Geek) that uses this wheeled multi-head sprayer to thoroughly clean the underside on the car, which I thought was a great idea.
https://www.amazon.com/Mingle-Pressure-Washer-Undercarriage-Cleaner/dp/B07K22DGRYAnd he also has his own line of detail products, he's gotta have one for road salt removal.
https://detailgeekautocare.com/collections/allThat's all I've got, mate!