Head Protector is required for Kumite up until Cadet - 15 YO (inclusive). It is optional for any further age group. It is best to see each tournament’s policy.

WKF Events require WKF-Approved Head Protectors.
NKF Events require NKF or WKF-Approved Protectors, but allows not-certified equipment as well.
AAU, WUKF Events require WUKF-Approved head gear, but they are generally flexible as long as it is not WKF.

There are many brands that are approved for WKF - so the models here are not the only ones you are allowed to pick.