This is Ren from the Shopify support team, here to help. Thanks for your post!
Currently, when an app is removed from the store, there is generally some code left behind, though newer apps are working to resolve this with a system that removes code along with the deletion of the app.
In the event that some code was left behind, such as in your case, you have a few options. You can try using a different version of your theme if you don't have other apps installed and don't mind some general setting up work or installing a new theme altogether. If the app wasn't used but rather just installed and uninstalled, you can also roll back to the previous version of your theme.
From your Shopify admin, go to Online Store > Themes.
Find the theme you want to edit, and then click Actions > Edit code.
Under the Templates header, click into one of the .liquid files you have edited recently.
Click Older versions next to the
.liquid file name
Click the drop-down menu to select an older version. Clicking a datestamp will roll the code back to that saved version.
Other than changing the theme or rolling back to a previous version, reaching out to the app dev would be your best bet. You can find their contact information in the App store to the right on the specific page of the app in question.
Let me know if I can help clarify any of that!