How do you create in realtime pixelated shadows - or convert Unity shadows to Doom shadows to create the pixelated effect?
I’m not sure what you’re exactly looking for, but as other have said, Doom doesn’t have shadows at all. It just has a primitive lighting model. I’m not even sure if they do a normal calculating. In Doom all environment is actually lit by baked lightmaps (or already baked into the textures). There are just a few actual dynamical lights. One light moves with the player. Also each shot emits a short light-flash. Besides that nothing of the lighting changes dynamically. Specular highlights are baked and don’t change when you move.
The reason why it looks “pixelated” is because it’s a quite low resolution compared to nowadays hardware.
If you want such a bad-resolution-effect, the easiest way is using a rendertexture like @screenname-taken already mentioned.