Correction: thought Outlook "old" and Outlook "new" were two distinct software packages. turns out it's the same! you can switch it by going to the "Outlook" menu. reverting my OL 16.58.22011705 it re-gains it's capability to work with Exchange Server on-premises AND showing a working index :))

Updating to 16.58.2210904 DID NOT help as our mailboxes are on on-premises Exchange servers which are not yet supported by New Outlook! If you try to add your account you'll be greeted by the wizard saying that Exchange Support would be "coming soon"

delete this!