check the last lines as well
Legacy API user identifier available on Basic Display API until September 30, 2020
To assist with migration, we have temporarily made the unique user identifier from the Legacy API - ig_id - available on Basic Display API. However, the ig_id field will be deprecated on Basic Display API on September 30, 2020, so please ensure you are using other fields (e.g. the Facebook "id" field) or your own unique identifier for your users. See our developer documentation here to learn more.
will be deprecated on Basic Display API on September 30, 2020