Shopify Apps on Checkout
No app is allowed to run in Shopify's checkout due to security reasons associated with PCI compliance. This has always been Shopify's policy, and even merchants subscribed to Shopify Plus (Shopify's enterprise tier) that can freely customize their checkout can't use standard apps in that part of the storefront. Check out the resources below for more information:
In the past, Shopify apps were able to modify the checkout by being added manually to the Additional Google Analytics Scripts. However, on January 30th, 2019, Shopify decided to block this as well.
Due to security concerns, this field was modified so that by default, scripts added to this field can’t be used to change the look or function of Shopify’s checkout. [...] In order to ensure that Shopify's App Partners provide a working experience to all users; app developers/partners whose apps make use of this field have been instructed to prepare and update their apps and documentation, as checkout customizations that rely on this field will no longer work for all merchants going forward.