I’ve just checked at https://developers.facebook.com/ and there is now an extra input box which isn’t shown in the instructions at step 5 of Configuring Facebook login for Discourse.
It is underneath “Privacy Policy URL” and is called “User Data Deletion”. It lets you enter a URL for “Data Deletion Callback URL” or “Data Deletion Instructions URL”. The prompt in the box says “You can also provide a link”. My box is blank and I don’t propose to change anything in case I break it…
A tooltip states: “The General Data Protection Regulation (GDPR) requires developers to provide a way for people to request that their data be deleted. To be compliant with these requirements, you must provide either a data deletion request callback or instructions to inform people how to delete their data from your app or website. Learn More”