Problem statement
During a deployment using the Auth0 Deploy CLI, we got the error message
Additional properties not allowed: new_universal_login_experience_enabled' on property flags
Solution
This new_universal_login_experience_enabled flag had previously been deprecated. And its removal should have no impact on your tenant. The sudden onset of this error in the CLI was due to a recent update in your environment to the Auth0 Management API (which is leveraged by the Deploy CLI), which removed this flag. You can use the following link to see the current flags available for configuring your tenant(s):
https://auth0.com/docs/api/management/v2#!/Tenants/patch_settings