All traveller’s fields on checkout are required or not.

Source

File: /inc/cart/checkout.php
Used by: /inc/cart/checkout.php

Parameters
Default is false.

Example:

function prefix_wp_travel_require_all_travelers_fields() {
    return true;
}
add_filter( 'wp_travel_require_all_travelers_fields', 'prefix_wp_travel_require_all_travelers_fields' );