Field type can be

• "modifiedbiomesource:minecraft_multi_noise" (Multi Noise)
• "modifiedbiomesource:minecraft_the_end" (The End)

There are mod_support, allows, denies, and fallback optional configuration. Field allows and denies can be biome or biome tag and field mod_support control mod support from compat.

{
    "type": "minecraft:overworld",
    "generator": {
        "type": "minecraft:noise",
        "biome_source": {
            "type": "modifiedbiomesource:minecraft_multi_noise",
            "preset": "minecraft:overworld",
            "mod_support": true,
            "allows": "#minecraft:is_forest",
            "denies": "minecraft:taiga",
            "fallback": "minecraft:forest"
        },
        "settings": "minecraft:overworld"
    }
}

Should at least set one of optional configuration excluding fallback or just vanilla biome source. Field mod_support doesn't do anything unless compat mod installed that will published later.