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

{
  "feature": "minecraft:patch_crimson_roots",
  "placement": [
    {
      "type": "minecraft:height_range",
      "height": {
        "type": "minecraft:uniform",
        "max_inclusive": {
          "below_top": 0
        },
        "min_inclusive": {
          "above_bottom": 0
        }
      }
    },
    {
      "type": "minecraft:biome"
    }
  ]
}