If you choose to use/switch to the New Outlook, shared mailboxes do not appear in the same way as in the classic Outlook.
The steps below will guide you to locating shared mailboxes:
Launch New Outlook.
Scroll down the list of your email folders and you should see a folder named "Shared with me".
Scroll down the list of your email folders and you should see a folder named "Shared with me".
Click on the arrow next to it to expand the contents and you should see your shared mailboxes.