Until Registry v3.1.0, the standard way for collecting a user's external identifier was via a semi-documented hack whereby the value of REMOTE_USER would be copied to the Petition's Org Identity record when Email Verification and Require Authentication were both enabled. This capability is deprecated as of Registry v4.0.0, and will be removed in Registry v5.0.0.
Because of the complex nature of enrollment flows, automated tooling will not be available to convert existing configurations. Administrators should review Consuming External Attributes via Web Server Environment Variables, and plan to adjust or replace any existing flows as necessary.
Additionally, keep in mind that Registry v5.0.0 will also remove the ability to create Org Identities manually using Enrollment Attributes of type Org Identity. Avoid setting up Flows with this sort of configuration when migrating.