I don't see any textures in Blender 3.0.1

[phax]

How often are these used?

Depends but these are not used that much

Aren't decal shaders used to put signs on walls, etc?

Decals are textures on top of another texture, like the dirt road in Cornfield Crossing

Is there a way to find all the places where something hasn't been exported into the new format and needs to be fixed manually?

I don't know, you'll have to check manually