Consumer vehicles have benefitted from improvements in the process to make safety glass, and are substantially more aerodynamic than they were.
This leads to much less bug splatter, which is good! But it exaggerates the decline as well.
In the 41 years since our VW Vanagon was made, the aerodynamic profile has not changed. And yet, while not measured with accuracy suitable for a lab report, we clean far fewer bugs of thing than years prior. As in, previously one might have to stop at a gas station just to clean the bugs off the windshield. Whereas these days, I'm actually surprised when a bug splats on the windshield.
That, and your premise is flawed to begin with: ...data showing that modern cars hit more bugs, perhaps because older models push a bigger layer of air – and insects – over the vehicle.
https://www.theguardian.com/environment/2020/feb/12/car-spla...
Also, bumpers and grills were covered in moths, butterflies and grasshoppers, etc... when I was a kid. Not anymore, not for a long time.
They are almost all gone now, clear as can be.