minecraft-assets/data/minecraft/worldgen/placed_feature/red_mushroom_old_growth.json

{
  "feature": "minecraft:patch_red_mushroom",
  "placement": [
    {
      "type": "minecraft:rarity_filter",
      "chance": 171
    },
    {
      "type": "minecraft:in_square"
    },
    {
      "type": "minecraft:heightmap",
      "heightmap": "MOTION_BLOCKING"
    },
    {
      "type": "minecraft:biome"
    }
  ]
}