Modern UI does not enforce mandatory metadata
I know this has been raised before, and declined by the SharePoint Team experience, however, if you look at this from the Security and Compliance and eDiscovery angle, and from a Records Management perspective, that metadata MAY be required and used for retention and sensitivity.
I think it is extremely narrow minded to simply disregard this option in the Modern UI.
Please bring it back. We need it!
Troy Phillips commented
This appears to have changed as of yesterday on my tenant. Have not seen this documented anywhere though...