Outlook - Restore the Unread Mail folder (Outlook 200x/201x/365)

Here it is Microsoft official article that instruct you on how to re-enable "unread email" folder in Outlook 200x/201x/365 in case it is no longer available.

https://support.office.com/en-us/article/restore-the-unread-mail-folder-a51a806f-9b83-4fe8-87d2-f1d7c1f55815

Tip: You can also open the New Search Folder dialog box by pressing CTRL+SHIFT+P.

Here they are some screenshot about resolution steps done on Office 365: