Hi @Beone - thanks for getting in touch. Per our documentation here, that endpoint is still supported, so no worries there. Since your app is still able to retrieve the old orders, it seems like there’s nothing wrong with your app’s access credentials either. Have you recently added a new fulfillment location for products/inventory items through the merchant admin or otherwise?
If you need to access the assigned fulfillment orders endpoint for a new location, you would need to set up the fulfillment service through this endpoint and then assign inventory to that location. If there have been no changes to the shop’s locations - no worries. If you’re able to provide an X-Request-ID from the response body payload from Shopify for one of the API calls, we can definitely look into this further for you.
Hope this helps - let us know if we can look into this further.
Al | Shopify Developer Support