The feature once installed and setup is simple to use. Edit a product and tick a check box that tells Drupal Commerce the product is no longer available.
The obvious questions would be, why not just delete the product or just zero the stock and do nothing.
The obvious answers to these to obviously stupid solutions would be, why bother running an online retail business that annoys existing customers and potential new customers!
Rule number one, provide your website users with the best possibly experience. If you had been selling a range of products for a long time and they stopped being made you will still attract users looking for them. During the time they were available they would be found by search results. Why would you want to stop people finding your website and alternative range of products?
So, you simply tick a box that in turn displays a message - 'this product is no longer available as it has been discontinued'
You then select the products that are very similar to the discontinued product as alternative options. They may not meet the requirements for the customer but you have made an effort and provided a better experience other than making the customer guess!
Would you like the DISCONTINUED Product feature for your Drupal Commerce website?
This is not a standard feature in Drupal Commerce. This is a bespoke module that can be easily installed on Drupal 8 and above. Its not technically related to the stock notification feature but is part of the general stock management best practice system. Managing stock for multiple items can be very time intensive. There is no Drupal Commerce built in feature that allows online stores with 1000's of items to manage stock easily so we built a feature to manage stock.