This document describes the deletion of DKIM keys in Email Security.cloud.
Email Security.cloud
Instructions on deleting the DKIM keys in Email Security.cloud.
Navigate to Platform > Outbound DKIM Signing Settings.
You can see that it is not possible to delete all DKIM keys even when DKIM is disabled for the domain. This behavior is by design in ESS.
The issue is that the system validates that an active selector cannot be deleted. When only one selector exists, it is automatically marked as active, since the selector choice is implemented as a radio button (meaning one selector must always be active).