The discount filter widget is available in WordPress Appearance > Widgets. Look for ‘Discount’ and select the Discount Filter:
After you select the filter you will see the options:
You can set the widget title and the label for the checkbox. Save the widgets.
Now you can see the discount filter on the widget area in the front-end. For example inside the catalog filters bar:
Please note that the checkbox will not show up if there are not products with a discount.