The form view is defined in website_sale.address and my v13 community and enterprise testing shows that the checkout flow cannot proceed without the State being selected. The form view does this with an error coded into the view. See my v13 snippet here.

Also note that the mandatory field requirements are handled in an additional layer via the following function in the v13 code in main.py:

def _get_mandatory_shipping_fields(self):

        return ["name", "street", "city", "country_id"]

Updating this code requires development and cannot be done via the front-end.