More accurately: do not store your data in a proprietary format. The issue isn't that the app stopped working, the issue is that the app changed from a single-

More accurately: do not store your data in a proprietary format. The issue isn't that the app stopped working, the issue is that the app changed from a single-purchase to a subscription model, and the last version of the single-purchase app is no longer supported. If the data was freely accessible, you could at least pull it out and migrate to an app with a more user-friendly business model.