Yes, the Mass File Migrate app is fully compatible with Salesforce Classic. Follow the steps below to use the app in the classic environment:
- Navigate to Salesforce Classic.
- In the apps dropdown located in the top-right corner, look for the "Mass File Migrate" app.
- Click on the app name to select it.

- Once clicked, the Mass File Migrate app will open within the Salesforce Classic environment.
- By following these steps, users can seamlessly utilize the Mass File Migrate app's features and functionalities in the Salesforce Classic interface.

PS: View Mass File Migrate Salesforce file migration app on Salesforce AppExchange:
https://appexchange.salesforce.com/appxListingDetail?listingId=e9874a66-2001-4e3d-8a29-3a2ed7f584c4