It seems really strange and everything points to a bug considering Postal Code used to be a mandatory field in the past. I was in contact with Shopify Support multiple times and was told that this was expected behaviour.
For us, this is a big deal breaker as a local payment gateway need this info to be mandatory for risk & compliance reasons (chargeback) so it currently forbids us approving our Online Shop for them.
Any chance that this will be fixed in Shopify's Core anytime soon?
We could not find a solution for this really - we managed a workaround via our CMS and pre-filled the Shipping Adress in the Checkout via Storefront API calls, thus being able to fetch the mandatory address info beforehand. This helped our acquirers validation process, but it still leaves the checkout field for postal code without actual validation during checkout in case someone would go and remove the pre-filled data from there - i suppose this is not a viable way for most real life use cases though. I really hope this will be fixed some time soon in the Shopify Checkout Core functionality.
This is an issue for us also. Having some form of validation on the zip code field to not be blank is needed at a minimum. If using the Local Delivery shipping method, there are options to only allow specific Zip codes, however without any frontend validation on the form, this allows the customer to get to the Shipping page when no shipping methods are available for their zip code.