To be compliant with General Data Protection Regulation (GDPR), applications that access user data must do one of the following:
Implement a Data Deletion Request Callback.
If your app type is Consumer or Gaming, either the callback or the URL is required for App Review and for Live mode.
The data deletion callback is called whenever an app user removes your app and requests that you delete their data. Your app users can do this by going to their Facebook profile and clicking the Send Request button on the Settings & Privacy > Settings > Apps and Websites page.