This filter will add SKU numbers to your WooCommerce emails in v2.6 and above

woocommerce-add-sku-to-emails.php

// Edit order items table template defaults
function sd_add_wc_order_email_sku( $table, $order ) {
  
	ob_start();
	
	$template = $plain_text ? 'emails/plain/email-order-items.php' : 'emails/email-order-items.php';
	wc_get_template( $template, array(
		'order'                 => $order,
		'items'                 => $order->get_items(),
		'show_download_links'   => $show_download_links,
		'show_sku'              => true,
		'show_purchase_note'    => $show_purchase_note,
		'show_image'            => $show_image,
		'image_size'            => $image_size
	) );
   
	return ob_get_clean();
}
add_filter( 'woocommerce_email_order_items_table', 'sd_add_wc_order_email_sku', 10, 2 );