Hello! I can now delete all business concepts (glossary terms, critical data elements, and OKRs) as well as domains themselves (now called "governance domains") in the new GA release of Microsoft Purview Data Governance. Additionally, I can also unselect the parent domain from a child domain.
You can see a list of the changes here: https://learn.microsoft.com/en-us/purview/whats-new#microsoft-purview-data-governance-ga
Additionally, you can see when your region will get this new GA update here: https://learn.microsoft.com/en-us/purview/data-catalog-regions