We are currently building a use case where users can apply loyalty points to a cart from a checkout ui extension and are looking to secure the flow. Anyone can update cart attributes, and customer meta fields are not fulfilling our needs since we have to support guest customers.
The 2023-04 API version brought meta fields to the cart resource
https://shopify.dev/changelog/add-metafields-to-cart-api but they haven't made their way into Shopify Function inputs.
Anyone nows if they would be added there in a near future?
While we don't currently provide direct support for Functions in this forum, the public [Shopify/function-examples GitHub repository] has an active [discussion board] that might be worth checking out.
Hope you have a great day