Using older versions of Outlook for Mac for personal email and calendar
Hi,
Unfortunately, even if you accidentally remove the @outlook.com email address or alias from your account, the removal is permanent. It cannot be added back to the account or reused.
Microsoft has published an article explaining this policy: https://support.microsoft.com/office/add-or-remove-an-email-alias-in-outlook-com-459b1989-356d-40fa-a689-8f285b13f1f2
I took a screenshot of that part of the article for you. Please see below: