In early 2022, Microsoft began pushing an updated version of Outlook for Mac called New Outlook. Some may see a banner in Outlook prompting for the new version or advising that 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.
There are legacy Outlook functions that are not yet compatible with New Outlook, per Microsoft they are in development and should be available later in 2022. These include: Contact Lists, export and import of account data (.olm and .pst files), and local "On My Computer" folders. Please see Recent Updates towards the bottom of Microsoft's The new Outlook for Mac page for the most current information.
If you use New Outlook and receive a warning regarding not being able to continue connecting to an existing Shared Mailbox, please try restarting Outlook; in most cases it will automatically reconnect to Shared Mailboxes when it reopens. You can also manually add them back if they do not reconnect automatically.
For more details please see Microsoft's website: