The app script is still in my template after the removal of the app

Before uninstalling the app, we always suggest disabling the app first to ensure that all traces of our code are also removed from your store templates.

If you disabled the app before it was uninstalled, you don't have to worry about any codes from our app. However, if you didn’t disable the app prior to uninstalling it, we cannot remove the app script from your store as the Shopify uninstall process does not allow us to access your store once you hit the 'Uninstall' button.

However, if you didn't disable the app before uninstalling it, you will find, at the end of your  theme.liquid template, a line similar to:

<script src="//video-background.incubate.dev/js/core/main.min.js?timestamp=1614709223&shop=yourstore.myshopify.com" defer></script> 

It won't harm your store, but you can remove it safely; just delete that line from your template.