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Theme app extension requirement - all new apps
Shopify App Store
Effective September 07, 2021
Starting Tuesday September 7, 2021, if you're submitting an app that integrates with a theme to the Shopify App Store, then you need to use theme app extensions.
Existing apps are strongly encouraged to adopt this feature as well. However, there is currently no deadline for existing apps in the Shopify App Store to adopt theme app extensions.