You can now read cart metafields in the GraphQL input query for Shopify Functions. Checkout UI extensions can also read and write cart metafields for abandoned carts, order edits, and draft orders, in addition to previously supported resources. Notably, cart metafields are carried over to abandoned carts for later use.
This is a companion discussion topic for the original entry at https://shopify.dev/changelog/cart-metafields-are-accessible-in-shopify-functions-and-checkout-ui-extensions