This UK study used number plates, which haven't changed form between the two years (2004 and 2021), and have found a decline of just under 60%. Aerodynamics may have something to do with it, but I know from driving through insect-abundant areas recently that you still get plenty of splatter in modern cars.
The number plate is still attached to the car and thus the shape of the rest of the car affects how air moves over it.
How much is hard to say. Aerodynamics is just hard but basically as the car moves through air it pushes air out of the way and this air also pushes other air out of the way and thus elements next to each other will effect each other.
How much is hard to say. Aerodynamics is just hard but basically as the car moves through air it pushes air out of the way and this air also pushes other air out of the way and thus elements next to each other will effect each other.