Costco sent out notice to members that after November 15, 2023, the online Costco.com will no longer accept Discover cards. As before, Costco clubs in-store only accept Visa cards. At Costco.com they previously accepted Visa, Mastercard, and Discover; going forward, only Visa and Mastercard will be options for online payment. You can see the full list of accepted payments at this Costco link.
Hat tip to DDG