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Any attributes and classes added to Active Directory cannot be removed. However, they can be defunct when they are no longer needed.

It's a best practice to keep the Schema Administrators group in Active Directory as empty as possible for as long as possible. When you're done, remove any accounts that you may have added to the Schema Administrators group.

If you want your Active Directory schema extension attributes to extend into Azure Active Directory as well, make sure it's a single-valued attribute.