Minecraft wiki entry for Old, associated text: Mineshafts, ocean monuments, woodland mansions, strongholds, dungeons, desert pyramids, jungle pyramids, swamp huts, igloos, ocean ruins, shipwrecks, and pillager outposts do not generate, making resources limited.
Caves and canyons do not generate in this world type, making the underground area solid, although hollow areas occasionally appear.
Tall grass and ferns did not generate as part of any biomes, though they could generate as part of village structures. This made certain biomes such as plains and savannas less grassy. Grass only generated in bamboo jungles, while large ferns were generated normally.
Even with experimental gameplay enabled, the world height was 128 blocks, with the bottom layer at Y=0.