Extend the output of the WooCommerce REST API for order line items

functions.php

<?php

/**
 * Example: Add order meta to the REST API
 * WC 2.6+
 *
 * @param \WP_REST_Response $response The response object.
 * @param \WP_Post $post Post object.
 * @param \WP_REST_Request $request Request object.
 * @return object updated response object
 */
add_filter( 'woocommerce_rest_prepare_shop_order_object', 'yanco_wc_rest_api_extend_order_lineitems_data', 10, 3 );
function yanco_wc_rest_api_extend_order_lineitems_data( $response, $object, $request ) {

    if( empty( $response->data ) ) {
        return $response;
    }
        

    $order_data = $response->get_data();
	
	foreach ( $order_data['line_items'] as $key => $item ) {
        $_product_id = wc_get_order_item_meta( $item['id'], '_product_id', true );
        $_stock_status = get_post_meta( $_product_id, '_stock_status', true );
        $_stock = get_post_meta( $_product_id, '_stock', true );

        $order_data['line_items'][ $key ]['stock_status'] = $_stock_status;
        $order_data['line_items'][ $key ]['stock'] = $_stock;
	}
	
	$response->data = $order_data;

    return $response;
}