Goal: upgrade the checkout flow (Information, Shipping, Payment) to Checkout Extensibility while keeping the legacy Thank you (order status) page to continue using Additional scripts.
Approach shared: use a draft checkout configuration (a staging version separate from the live checkout). When publishing the draft, choose “Checkout pages only” instead of “All pages” so the Thank you page remains unupgraded. A Shopify help doc on live vs. draft checkouts was provided and used.
Outcome: The original poster confirmed this method worked and they can keep using Additional scripts until they are deprecated next year.
Open issue: Another participant who already upgraded all pages asks if duplicating their live (fully upgraded) configuration means the draft also has all pages upgraded, and whether it’s possible to get back a non‑upgraded Thank you page. This remains unanswered; the thread is not fully resolved.
According to the Checkout Extensibility upgrade Guide here:
You can upgrade the information, shipping, and Payment pages without upgrading the Thank you page. It of course does not explain how to do this. When I go to checkout settings, it only gives me one option to upgrade everything. I don’t see an option to only upgrade the information, shipping, and Payment pages without upgrading the Thank you page so that I can continue to use additional scripts. What am I missing?
It is concerning that I posted this seemingly simple question 9 days ago and not one Shopify “support” person can answer it. It’s hard to support something that is eternally in BETA, I suppose.
To my understanding, you need to create a draft checkout first. You can choose to upgrade all pages (including Thank you page) or only Checkout pages when publishing the draft checkout.
Thank you for the reply. What are the steps to accomplish what you are describing? I’ve never heard of a draft checkout.
Check this out: https://help.shopify.com/en/manual/checkout-settings/customize-checkout-configurations/live-and-draft-checkouts
Thank you so much. It worked perfectly. Now I can continue to use the additional scripts until they deprecate it next year. 
If I’ve already upgraded all pages when published - my live configuration has all upgraded pages. So duplicating that, the draft will have all pages upgraded, right? Am I now unable to get a non-upgraded thank you page?