As of Storefront API's 2024-04 release, you can use
buyerIdentity.deliveryAddressPreferences.deliveryAddressValidationStrategy (accepted values
COUNTRY_CODE_ONLY) to enable address validation on the passed
deliveryAddress for any cart mutations that accept a buyer identity as input (like
STRICT will perform a full validation on the address fields, and will raise
CartUserErrors for invalid addresses. Invalid addresses will not be stored on the cart.
COUNTRY_CODE_ONLY or omitting the field will only validate the presence of a valid country in the address (unchanged behavior from previous version).