On my Mobile app, under Autopay is the following statement:
"As early as May, customers on AutotPay eligible plans will be required to use a debit card or bank account as their payment method to receive the monthly AutoPay discount. Change the Payment method >"
Edited to add: This statement now appears on my desktop Account Manager Dashboard, under the hyperlink-AutoPay, mid page.
I don't know when that was added since I seldom use the Mobile app. Screen Capture from my iPhone 14 Pro Max.