Maybe "Classic" would be a better term than "Old School". It's a term that describes the sort of map that we gushed over from TSR and the like. Ruins of Undermountain etc. The look of being drawn on graph paper. The more "cutting edge" maps around now tend to try for a "hand-drawn", "artistic" or realistic (actual map) look. Then, there's the 3D lot, who model the dungeons.