Not sure where to post this.
I don't see what functionality managed metadata brings to the Modern user interface.
All of the following features are lost:
1) Managed Metadata Navigation that provides a controlled vocabulary for search is not supported.
2) Search does not return results that are in context. One of the reasons to have metadata is to provide context to the documents. If I search on a term I get results that provide every document in my library that contain that term. The documents that have metadata that contain the term are not prioritized, I.e. that could be half way down the search results list. The only way to find them is to scroll down the list until the term appears in the metadata.
3) Required fields are not required.
4) Synonyms are not supported.
The Modern interface may be fine for small repositories, however, if SharePoint is going to be a viable contender in areas like Accounts Payable, Accounts Receivable, HR, Research, etc., the functionality that was provided in the Classic interface is going to have to be available in the Modern interface.
Until then I will use the Classic user interface.
Jim Wade