fz-survival-datapack/data/fz/advancements/unsneak.json

{
    "parent": "fz:sneak",
    "criteria": {
        "unsneak": {
            "trigger": "tick",
            "conditions": {
                "player": {
                    "flags": {
                        "is_sneaking": false
                    }
                }
            }
        }
    },
    "rewards": {
        "function": "fz:event/on_unsneak"
    }
}