Using Classic Outlook on Windows for personal email, calendar, and contact management
As Roady [MVP] said in this thread
'And that's exactly why New Outlook will be in development for a long (long!) time before it will reach the general "Enterprise Ready" state and Classic Outlook will be around until at least (!) 2029.
The Shared Mailbox experience is currently being worked on and the first iteration of it is currently scheduled to start rollout (which isn't the same as 100% availability!) in July.
See; Outlook: Shared Mailboxes as Accounts | Microsoft 365 Roadmap'
So its on its way - just not yet.
I/We are still recommending people with shared mailboxes to use the classic/old Outlook for now.