minecraft-assets/assets/minecraft/advancements/story/elytra.json

{
    "display": {
        "icon": "minecraft:elytra",
        "title": "Sky's the Limit",
        "description": "Find an Elytra"
    },
    "parent": "minecraft:story/enter_end_gateway",
    "criteria": {
        "elytra": {
            "trigger": "minecraft:inventory_changed",
            "conditions": {
                "items": [
                    {
                        "item": "minecraft:elytra"
                    }
                ]
            }
        }
    }
}