Register custom taxonomy on woocommerce product and add to REST API

gistfile1.txt

/*
  Simple Taxonomy */
add_action( 'init', function() {
	register_extended_taxonomy( 'customtax', 'product' ); // This relies on https://github.com/johnbillion/extended-cpts/ 
} );


//Register taxonomy API for WC
add_action( 'rest_api_init', function() {
	register_rest_field('product', 'customtax', array(
		'get_callback'    => 's9_product_get_callback',
		'update_callback'    => null, // 's9_product_update_callback',
		'schema' => null,
	));
} );

//Get Taxonomy record in wc REST API
function s9_product_get_callback($post, $attr, $request, $object_type) {

	$terms = array();

	// Get terms
	foreach (wp_get_post_terms( $post[ 'id' ],'customtax') as $term) {
		$terms[] = array(
			'id'        => $term->term_id,
			'name'      => $term->name,
			'slug'      => $term->slug,
			'custom_field_name'  => get_term_meta($term->term_id, 'custom_field_name', true)
		);
	}

	return $terms;
}

//Update Taxonomy record in wc REST API
function s9_product_update_callback($values, $post, $attr, $request, $object_type) {
	// Post ID
	$postId = $post->get_id();

	//Example: $values = [2,4,3];

	// Set terms
	wp_set_object_terms( $postId, $values , 'customtax');

}