From what I understand, Instagram login can’t be achieved anymore and has been replaced by Facebook logins.
The related settings in Discourse should be then removed unless they have other use (like twitter rich embed)
Note that Basic Display is not an authentication tool. Data returned by the API cannot be used to authenticate your app users or log them into your app. If your app uses API data to authenticate users, it will be rejected during App Review. If you need an authentication solution, use Facebook Login instead.
Yes, Instagram no longer encouraging the login feature and the existing code is not working. So the above site settings are removed in the below PR.