Nothing.
The app does not collect, store, transmit or share any personal data, usage data, device identifiers, location, or analytics of any kind. It contains no advertising, no analytics library, no crash reporting service and no account system.
The app does not request the Internet permission. It has no ability to send data anywhere.
The plans you draw are stored only on your device, in the app's private storage. They are deleted when you uninstall the app.
If you choose to export a plan (as a PDF, PNG or plan file), the file is written to your device's Downloads folder and is under your control from then on. If you choose to share an export, it is handed to the app you pick through the Android share sheet; Plotline itself never uploads it.
The app collects no data from anyone, including children.
If this policy ever changes, the change will be noted here with a new date. Given that the app has no network access, any change that involved collecting data would also require a new app version requesting new permissions, which you would be asked to approve.