How would a DMZ zone recertification process look like? Say we create a separate security zone for each application hosted in DMZ. Now we would like to get in touch with the person in charge for the given zone to ask if it's still needed. How could this be handled? Are there any ISO 27001 directives related to this?
What you are asking for is basic lifecycle management:
You need a policy and a procedure to review all the firewall configs (including the zones) to ensure that the current 'design' is meeting the needs and goals of its intent. The frequency of this review is up to the needs of your organisation, yearly might be fine.
Each 'zone' requires an 'owner' who designs and drives the controls in this zone. Any changes to the zone require the owner's input. Ownership is set up as early as possible to prevent the owner being surprised that they are responsible for critical infrastructure.
During the review phase, each relevant element of the zone is verified by the owner as being current, and the verification is challenged by an outside party (GRC, Information Security Steering Committee, etc.) as being accurate.
Once verified or modified, the zone returns to a 'maintenance' state. If the zone is deemed redundant, a decommissioning phase is initiated which defines and tests the removal of the zone in order to ensure a complete and secure decommissioning with minimal impact to the organisation.
As for ISO 27k, the terms you are looking for a 'controls review', and there is a whole document about this: ISO 27008