Cart tokens will now be returned with a new format for both the AJAX and Storefront GraphQL Cart APIs. API features and functionality remain unchanged. Apps and themes should be designed to handle cart tokens in any format and of any length. Treat the cart token as a random identifier that will change in the future.
This is a companion discussion topic for the original entry at https://shopify.dev/changelog/changes-to-cart-token-format-for-ajax-and-storefront-graphql-cart-apis