It finally works.
verified on store and also in documentation - https://changelog.shopify.com/posts/plus-merchants-can-now-use-apps-built-with-compatible-ui-extensions-on-draft-order-checkouts
Topic summary
Checkout UI extensions were not loading on draft order checkouts. Shopify staff confirmed earlier that draft/invoice checkouts weren’t supported yet, prompting concerns about the August deprecation of checkout.liquid and repeated requests for a timeline.
Recent update: After upgrading to API 2024-07 and implementing the required Cart Instructions API, developers report that UI components now render on draft order checkout. Shopify’s changelog indicates availability for Plus merchants (“apps built with compatible UI extensions on draft order checkouts”).
Current limitations/issues:
- Storefront API calls from draft checkout returned 403 (“Channel not found for the given token”). Shopify support acknowledged this as a bug and said a fix (PR) is in progress.
- instructions.attributes.canUpdateAttributes consistently returns false on draft orders. Support says this is expected for now because draft checkouts don’t yet support all checkout features; under active development.
Outstanding questions: Whether attributes/metafields/notes can be updated during draft checkout remains effectively blocked by canUpdateAttributes=false.
Status: Partially resolved. Extensions now render (for Plus) with API 2024-07, but functionality is limited and one known API bug is being fixed. Monitor the Shopify changelog for further updates. An image of the roadmap was shared but is not essential to the outcome.