Extend WP Rest API with `parent_category` param that includes posts from subcategories

functions.php

<?php 

// Add somewhere in functions.php for your child theme
// Add parent_category filter to REST API
if (!function_exists('wprabpc_wp_rest_api_by_parent_category')) {
    function wprabpc_wp_rest_api_by_parent_category($args, $request)
    {
        if (isset($request['parent_category'])) {
            $parent_category = sanitize_text_field($request['parent_category']);
            $args['tax_query'] = [
                [
                    'taxonomy' => 'category',
                    'field' => 'term_id',
                    'include_children' => true,
                    'operator' => 'IN',
                    'terms' => $parent_category,
                ],
            ];
        }
        return $args;
    }
    add_filter('rest_post_query', 'wprabpc_wp_rest_api_by_parent_category', 10, 3);
}