From now on, you can find info about applied discount coupons in dataLayer on completed order page.
You can find it under dataLayer[0]['shoptet']['order']['discountCoupons']; – empty array when no coupons were applied, or array containing applied discount coupons.
[
{id: 3,
name: "Slevový kupon č. 123456 - Sleva 10%",
code: "123456"}
]