New Outlook Becomes Default for Macintosh in Mid to Late February
Microsoft is changing the default version of Outlook on compatible Macs to "New Outlook" beginning in mid February, with the release of Outlook for Mac version 16.58. There is no exact release date, and you may see a banner in Outlook prompting for the new version or advising you it will launch the next time Outlook restarts. If you prefer to revert back you can simply select Help from Outlook's upper menu, then Revert to Legacy Outlook.
New Outlook has a sleeker look, moving away from the ribbon display to a simplified toolbar that can be customized. Some items from legacy Outlook's ribbon will now be found in the topmost menu (File, Edit, View...). It also has an improved search, event scheduling and RSVP, and more.
There are legacy Outlook functions that are not yet compatible with New Outlook, per Microsoft they are in development and should be available later this year. These include: Contact Lists and Folders, export and import of account data (.olm and .pst files), local "On My Computer" folders, Block sender and resend message.
Testing at CUIMC also showed that some received a warning regarding not being able to continue connecting to an existing Shared Mailbox, however New Outlook automatically reconnected to Shared Mailboxes after restart. They can also be manually added back if they do not reconnect automatically.
For current details please see Microsoft's website: