mcserver/instances/forge-1.20.1-survival/runtime/data/config/alexsmobs/farseer.json

{
  "biomes": [
    [
      {
        "type": "BIOME_TAG",
        "negate": true,
        "value": "forge:no_default_monsters"
      },
      {
        "type": "REGISTRY_NAME",
        "negate": true,
        "value": "minecraft:mushroom_fields"
      }
    ]
  ]
}