Foveon just has 3 vertically stacked layers of photodiodes (one for each RGB colour) instead of a single one with Bayer filters like other digital camera sensors.

It’s not related to film grain (which is a byproduct of silver particles in celluloid).

The behavior of the sensor with its flaws closely resembled film. The sharpest digital pictures I've ever observed.

they are very sharp indeed, and the colours are quite film like. I have a DPM2 and waiting for the Foveon full-frame sensor that Sigma is still working on.

I think the problem resolved itself when a new generation grew up with digital photography and did not long for grain. Now it is a retro aesthetic, like gen-Z's use of pixel art.