Maybe I'm being really thick, but why is this information that the OS would make available to apps?
Edit: It's not a last modified timestamp, it's a volume creation timestamp: https://github.com/mysk-research/loupe/blob/2262efd4456ecba8...
If you want someone to pay for an app, don't make it free with in-app purchases. This is not something allowing for the OS to provide a unique identifier that can be abused available to app developers. App developers cannot be trusted. At. All. Ever.