This is returning an empty array for me as well. A fairly simple workaround for anyone struggling with this broken hook is below:
Loop each of your line-items (useCartLines), calculate the total of each line-item applied discounts (cartLine.discountAllocations), that’s your total applied discounts.
https://shopify.dev/docs/api/checkout-ui-extensions/2023-07/react-hooks/cart/usecartlines
