minecraft-assets/data/minecraft/worldgen/configured_feature/pile_hay.json

{
  "type": "minecraft:block_pile",
  "config": {
    "state_provider": {
      "type": "minecraft:rotated_block_provider",
      "state": {
        "Name": "minecraft:hay_block",
        "Properties": {
          "axis": "y"
        }
      }
    }
  }
}