Though front engined cars continue to have a radiator in the same forward position as they did 100 years ago, in the 70s cars started to arrive with no visible front ventilation at all.
Examples are the original Ford Sierra, Porsche 944, Triumph TR 7, very rare Citroen SM and Honda Prelude.
Many more cars had no bonnet (hood) grille, but hid it under the bumper area like the Citroen ZX and first Rover 200/400 series, but even these could only get ventilation up into the engine area indirectly.
Others have tiny slits smaller than letterbox height like the Nissan 100 NX and Daihatsu Charade.

Considering a look under the bonnet will show a similar layout, though the space taken up by the machinery can be from approximately 50% for an old Ford Corsair, to a packed in engine, pipes, computer, electrics etc. in a modern Japanese sports car which hardly even seem to leave space for maintenance. So how do some cars still need traditional large grilles, as many do, but others have none or almost none? It seems to have no relation to the engine contents.

I think that when car manufacturers started closing the grille slots they found other ways of ducting air to cool the engines without sucking up rain water from the road.
I can't site specific engineering plans but they still have to provide ducted cooling. My truck and car have large grilles with the customary radiator being the first thing that moving air hits. With the advent of electric cooling fans instead of relying on engine belt driven fans and with transverse mounted engines to keep a low bonnet (hood) profile, cooling air for the engine was directed where needed with ducts. I think.

I think that makes a lot of sense, John. Strange the way some new designs retain the old size (which I far prefer) especially the Fiat Doblo van. In fact (as I've mentioned elsewhere) the Japanese have produced 'classic' versions of some normal cars for their home market for about 10 years, with traditional 1950's type chrome grilles and round headlights, such as the Daihatsu Mira Classic , one of my favourites, with 100 sold in the UK, and Chrysler are now toying with the idea as well.