As you know, most website visitors are mobile users. So it would be great if you add a simple but important new widget, so that users can easily return to the previous page using it.
This button is used in applications and some big websites like Instagram. Especially in store websites this will be really great and useful.
Imo swiping at the side of the screen is easier than using a back button and every phone I know has that interaction. If you still wanna add a button you can easily create one, save it as a template and insert it below your header or wherever you’d like.