minecraft-assets/assets/minecraft/models/block/deepslate_tile_stairs_inner.json

{
  "parent": "minecraft:block/inner_stairs",
  "textures": {
    "bottom": "minecraft:block/deepslate_tiles",
    "side": "minecraft:block/deepslate_tiles",
    "top": "minecraft:block/deepslate_tiles"
  }
}