AfterSell currently has limited compatibility with Thank You Page (TYP) extensibility. While product recommendations and storewide popup widgets are not yet available, other features like TYP content widgets are fully supported and compatible.
Thank you page offers are available on scripts and here are the steps to revert back to scripts, in order to setup Thank you page upsells:
Steps for the Shopify Plus plan:
You can revert just your Thank You Page to no longer use Extensibility on that page. To do so, you would need to go to your Shopify Settings> Checkout and click on revert and then follow the on-screen prompts:
Steps for non Shopify Plus plans:
Go to your Shopify Settings> Checkout, scroll to the bottom of the page and you can click on revert:
After reverting, you can go to the Thank You Page editor in your AfterSell funnel(s) and use the "Edit Scripts" option to set up your offers as usual.