Minecraft wiki entry for Stairs, associated text: Cobblestone and stone brick stairs generate naturally in strongholds.
Nether brick stairs generate naturally in nether fortresses.
Spruce stairs generate naturally in witch huts and in igloo basements.
Purpur stairs generate naturally in end cities.
Oak, dark oak and birch stairs as well as cobblestone stairs generate naturally in woodland mansions.
Oak, birch, spruce, jungle, acacia, and dark oak stairs can generate naturally in shipwrecks.
Dark oak, cobblestone, and mossy cobblestone stairs can generate in pillager outposts.
Blackstone and polished blackstone brick stairs can generate in bastion remnants.
Cobbled Deepslate, Polished deepslate, deepslate brick and deepslate tile stairs can generate in ancient city.‌[upcoming: JE 1.19]