Explanation/Reference:
Explanation:
Instead you should customize the default synchronization rule.
Note:
To delete a custom domain name, you must first ensure that no resources in your directory rely on the domain name. You can't delete a domain name from your directory if:
Any user has a user name, email address, or proxy address that includes the domain name.

Any group has an email address or proxy address that includes the domain name.

Any application in your Azure AD has an app ID URI that includes the domain name.

References:
https://docs.microsoft.com/en-us/azure/active-directory/hybrid/how-to-connect-create-custom-sync-rule