Discontinued Products for WooCommerce works by adding a “Discontinued” stock status to WooCommerce. If you deactivate or delete the plugin then this stock status is no longer available, so products can no longer be marked as discontinued.
On deactivation, all discontinued products are automatically marked as “Out of stock”. This ensures they remain non-purchasable. If you reactivate the plugin in the future, the products will revert to “Discontinued” status. (So if you make product changes while the plugin is deactivated, it’s a great idea to double check your stock settings when reactivating!)