As of July 28, 2025, "Sell only within configured shipping zones" has been enabled on nearly all shops at checkout. This change ensures that customers can only purchase products that are available at fulfillment locations configured to deliver to their shipping zone. This setting will continue to be removed up until September 30, 2025.
This is a companion discussion topic for the original entry at https://shopify.dev/changelog/removal-of-support-for-sell-from-all-locations-fulfillable-inventory-setting