We'll create fresh WordPress site with WooCommerce Order Pending To Cancelled Email installed. You have 20 minutes to test the plugin after that site we'll be deleted.
In previous versions of WooCommerce, when an order status changed from pending to cancelled, an email notification was sent automatically.
In WooCommerce 3.0.8, however, they’ve removed this (see changelog).
This (very) simple plugin brings back this email.
Thanks to @lomars for the code he shared on this Stack Overflow question.