Hi Shopify Addict here,
-In response to the whole thread-
The solution is available for Shopify plus : https://shopify.dev/docs/api/functions/reference/fulfillment-constraints/graphql/common-objects/mustfulfillfrom,
Using checkout ui extension allows to create a back in stock flow from there.
Nevertheless you are able to create a custom shipping method and link them to specific “Shipping origins” which are locations. Thus when locations become OOS you won’t have any shipping method available in the checkout.
If you want something more robust, you should probably use a server side integration on your PDP to query the desired location’s stock availability using Shopify ADMIN API and display the desired UI.
Best regards.