Essentially, the New Universal Login experience only offers progressive profiling instead of additional sign up fields. Only the Classic Universal Login experience allows for additional sign up fields.
This topic seems related to our new EA feature, Forms for Actions, which rolled out of the beta stage on May 7th. Our Product team prepared the whole Documentation page with examples of Use cases. Forms for Actions. If you find yourself having questions regarding this new feature for the next two weeks, we are hosting an Ask Me Anything session. Our Product Expert will provide comprehensive written answers on May 21st from 8 AM to 10 AM PST.
Find out more about Forms for Actions and learn more about AMA.