Using Classic Outlook on Windows for personal email, calendar, and contact management
I have been searching for an answer to this problem when I stumbled upon this support article which suggests it is possible to re-order mail folders in the new Outlook - Change the order of folders in the Folder Pane in Outlook - Microsoft Support
The "Custom folder order" doesn't exist, of course, so not only is there a basic function missing in new Outlook, but the support information is incorrect!