No doubt, there's a huge population decline. I didn't mean to detract from that. However, the primary splatometer for drivers is the windshield, not the license plate. The test put the splatometer on the license plate, which is a very reasonable place when employing the public as volunteer testers. So yes, older vehicles' license plates may catch slightly more bugs, but there's still a major difference in windscreen shape and upper aerodynamics.

If you look at, say, 2018+ vehicles, I'd say basically every reasonable passenger vehicle will have a subtle lower air splitter, a sharp protrusion at the lower edge of the bumper. This is meant to slice the air relatively cleanly to prevent bumper-level air from going under the car, instead going around and over. However, around 2018, upper splitters began appearing at the top of the bumper as well, in the form of sharp hood rims or grille features. The intent is to prevent air from going over, hitting the windscreen, and adding to the volume over the roof and upper wake, instead sending it to the sides where it finds a car's length of smooth side paneling. The 2018 Accord and 2016 Civic comes to mind, both a redesign that slanted the chrome grille bar above the logo forward. I'm not saying that every vehicle in the 2019 study was a 2018+ Accord and 2016+ Civic, just an example of ever-changing aerodynamic practices. One (or two) splitters will make the air more forceful on the bumper while greatly smoothing the total vehicle's airflow. Dial it back to 90s Fords, and the lower bumper was often rounded under the car in their bubble era. That works for airplane design, but is a negative feature on a grounded vehicle.

Unfortunately, there's no easy way to get a true splatometer measurement. You'd need, say, a license plate on stilts, vaulted 5ft in front of the vehicle to be outside the vehicle's aerodynamic influence. I suppose a net would also work and we don't actually need splats to count.