Save categories in order line item metadata

functions.php

add_action( 'woocommerce_checkout_create_order_line_item', 'custom_checkout_save_category_data_on_the_order', 20, 4 );

function custom_checkout_save_category_data_on_the_order( $item, $cart_item_key, $values, $order ) {
	if ( $item->is_type( 'line_item' ) ) {
		// Get the product categories for this item
		$terms = wp_get_post_terms( $item->get_product_id(), 'product_cat', array( 'fields' => 'names' ) );

		// Save a coma separated string of product category names in line item meta
		$item->update_meta_data( 'Categories', implode( ', ', $terms ) );
	}
}