When you create an external auth identity provider, a callback URL is generated for the external system to redirect back to. Now, the callback URL is constructed by using the org’s My Domain name. Instead of the URL https://login.salesforce.com/services/extidp/callback, the callback URL is now https://<my_domain_name>/services/extidp/callback. If the external auth identity provider is associated with a managed package, then the callback URL is constructed by using the package namespace.
Where: This change applies to Lightning Experience and Salesforce Classic (not available in all orgs) in Enterprise, Performance, Unlimited, and Developer editions.
Why: Salesforce is migrating global login endpoints from Salesforce-managed first-party data centers to Hyperforce. See Salesforce Express Connect (SEC) Users - Prepare for Global Login Changes.
How: After you create an external auth identity provider, you can find the callback URL in that auth provider’s detail page in the Identity Provider URLs section.