renewable-resources/data/renewable/recipe/elytra.json

{
  "type": "minecraft:crafting_shaped",
  "category": "misc",
  "pattern": [
    "M M",
    "MDM",
    "M M"
  ],
  "key": {
    "M": {
      "item": "minecraft:phantom_membrane"
    },
    "D": {
      "item": "minecraft:diamond"
    }
  },
  "result": {
    "id": "minecraft:elytra",
    "count": 1
  }
}