add_filter( 'wcfm_membership_registration_fields_address', function( $fields ) {
    if( isset( $fields['zip'] ) && isset( $fields['zip']['custom_attributes'] ) ) {
        if( isset( $fields['zip']['custom_attributes']['required'] ) ) {
            unset( $fields['zip']['custom_attributes']['required'] );
        }
    }
    return $fields;
} );