Minecraft wiki entry for Slab, associated text: Smooth stone slabs generate naturally in strongholds.
Sandstone slabs generate naturally in desert wells, and desert pyramids.
Spruce slabs generate naturally in igloo basements.
Oak, birch, spruce, smooth stone and cobblestone slabs generate naturally in woodland mansions.
Dark oak, cobblestone and mossy cobblestone slabs generate in pillager outpost.
Oak, birch, spruce, jungle, acacia, and dark oak slabs can generate in shipwrecks.
Blackstone and quartz slabs generate in bastion remnants.
Purpur slabs generate naturally in end cities.
Deepslate brick, deepslate tile, cobbled deepslate and polished deepslate slabs generate naturally in ancient city.‌[upcoming: JE 1.19]