I’m also facing this same issue.
I have been waiting for this feature since long. I got excited to see the documentation available for this some days back. I tried it by returning the PreUserRegistrationError error in the callback as shown in auth0-docs. So when I logged the error, it shows it in the logs. Sadly it’s still not showing on the UI.
I am already using the “Universal Login - New Experience” which is a prerequisite for this to work.
Please suggest if anything else I can try to make it work.