minecraft-src/data/minecraft/worldgen/configured_feature/mushroom_island_vegetation.json

{
  "type": "minecraft:random_boolean_selector",
  "config": {
    "feature_false": {
      "feature": "minecraft:huge_brown_mushroom",
      "placement": []
    },
    "feature_true": {
      "feature": "minecraft:huge_red_mushroom",
      "placement": []
    }
  }
}