Hi Orvil Ownby ,
Removal of any *.onmicrosoft.com domain is not supported today. Removing this domain can also have irreversible consequences to existing Exchange and SharePoint configuration if it was ever the primary in the past. If you need to remove this domain, the workaround is to create a new tenant with the desired *.onmicrosoft.com domain and migrate the users to that new tenant.
This is also documented in Add and replace your onmicrosoft.com fallback domain in Microsoft 365:
You are limited to five onmicrosoft.com domains, and currently they cannot be deleted once they are created.
Let me know if this helps and if you have further questions. If you would like to leave feedback to change this experience and have a strong business justification, you can do so in the feedback forum. https://feedback.azure.com/
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