After the recent changes at the Shopify platform we had to make some updates to our Shopify Feeds generator.
Shopify has deprecated its private apps. They were not completely removed, they were replaced by custom apps.
You can still continue using your existing private apps and its credentials with your Shopify feeds. But if you wish to create a new app you will need to work with a custom app.
If you have both private (legacy) and custom (new) apps, you can still use either one. When you authenticate your Shopify store, please use a switcher to choose which app’s credentials you wish to use: