Remove Windows Server from Domain While it is Airgapped?
Solution 1:
What problem are you experiencing? This should work fine.
Clone the server to an isolated network.
Remove the cloned server from the domain. You'll get an error that a domain controller cannot be contacted, but the cloned server will end up in a workgroup.
Sysprep it.
Convert to template.
Solution 2:
A domain client doesn't need to be connected to the domain in order to disjoin it. The only caveat being that the computer account in the domain will be orphaned and you'll need to delete it manually.