This is often referred to as never expiring products. It describes the process for what should happen when a product is deleted from the website. Websites usually have reasons for removing products from their site including change in season, out of stock, no more supplies, change in product ranges. However what is often forgotten about is people are still using the Internet looking for these "old" products. They may have bookmarked your site, searched for it in Google or emailed the details to a friend.
Rather than remove the product completely from the site, it is better for both users and search engines to leave the product still on the site, but explain to the user why it can no longer be purchased. These types of products are usually removed from competitor sites meaning there is less competition within search engines and more chance of attracting the user to your site. The sale can then be guided by the products that are available and in stock.