matrix-spec/api/client-server/v2_alpha/definitions/state_batch.json

{
  "type": "object",
  "additionalProperties": {
    "type": "object",
    "x-pattern": "$EVENT_TYPE",
    "additionalProperties": {
      "type": "object",
      "x-pattern": "$STATE_KEY",
      "allOf": [{"$ref": "event.json" }]
    }
  }
}