The Email settings for "New Order" the subject need to be (as in your question):
New Order - [{product_name}] ({order_number}) - {order_date}
In the code below I replace {product_name}
by the items product names (separated by a dash) as an order can have many items…
This custom function hooked in woocommerce_email_subject_new_order
will do the trick:
add_filter( 'woocommerce_email_subject_new_order', 'customizing_new_order_subject', 10, 2 );
function customizing_new_order_subject( $formated_subject, $order ){
// Get an instance of the WC_Email_New_Order object
$email = WC()->mailer->get_emails()['WC_Email_New_Order'];
// Get unformatted subject from settings
$subject = $email->get_option( 'subject', $email->get_default_subject() );
// Loop through order line items
$product_names = array();
foreach( $order->get_items() as $item )
$product_names[] = $item->get_name(); // Set product names in an array
// Set product names in a string with separators (when more than one item)
$product_names = implode( ' - ', $product_names );
// Replace "{product_name}" by the product name
$subject = str_replace( '{product_name}', $product_names, $subject );
// format and return the custom formatted subject
return $email->format_string( $subject );
}
Code goes in function.php file of your active child theme (or active theme).
Tested and works.
You will get something like this: