The supra does gather dirts in those areas, even worse if you run without wheel liners.
The front wing needs unbolting from underneath and cleaning now and again, likewise the rear areas gather the same dirt/ dust/ mud rusting the rear quarter and vertical seams.
When the car gets wet the gathered dirt gets saturated is in constant contact with the body, and will if never cleaned will probably end up like that I would imagine.
Nice.
Cleaning, treating, underseal with wax oil, additional wax oil coatings are your friends.
The average Joe who just uses a car as a car probably never ever thinks about doing these type of things, and nearly all rely on the annual mot for peace of mind.
As the supra seems to fair well during annual inspections, it could be many years for things to accumulate.