In June 2022, Shopify announced Checkout Extensions, a new way to customize the Shopify Plus Checkout. This new checkout is also referred to as ‘Checkout Extensibility’ or ‘Checkout One’
Since October 2022, existing Shopify Plus merchants already could request access to Checkout Extensions, and new merchants would by default get started with Checkout Extensions.
Now, in January 2023, the move is officially happening. New and existing Shopify Plus merchants can no longer request access to checkout.liquid if they didn’t already got access to it before.
Merchants requesting to enable checkout.liquid receive the following reply from the Shopify Plus team.
"As your checkout on yourstore.myshopify.com has been upgraded, you don’t have access to checkout.liquid file anymore.
On the new version of the checkout, you can make customisations via installing apps from the Shopify’s Apps store or developing your own custom apps.
The apps can be then added and configured in the checkout editor of your store: Settings > Checkout > Checkout customizations and branding > Customise.
Shopify partnered with UpsellPlus to launch Checkout One. We currently support both checkout.liquid and Checkout Extension type checkouts on Shopify Plus, so whichever type you have enabled, we’re ready to help you launch checkout upsells.
Here you can find the Shopify roadmap for Checkout One.