fz-survival-datapack/data/fz/advancements/event/on_sneak.json

{
    "criteria": {
        "on_sneak": {
            "trigger": "minecraft:tick",
            "conditions": {
                "player": {
                    "flags": {
                        "is_sneaking": true
                    }
                }
            }
        }
    }
}