When you uninstall the application in the Shopify admin panel, the theme will still contain code snippets from the application.
This is typical for all Shopify applications because after you uninstall the application from your store it no longer has the rights to restore all the previous changes it made to the theme.
Remove the code highlighted in the screenshot below located in (
customers/register.liquid):
2. Remove the following file:
customers/register-without-spurit_upsell_by_email.liquid