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

{
  "feature": "minecraft:iceberg_packed",
  "placement": [
    {
      "type": "minecraft:rarity_filter",
      "chance": 16
    },
    {
      "type": "minecraft:in_square"
    },
    {
      "type": "minecraft:biome"
    }
  ]
}