To erase your data, delete your account in the app’s settings before uninstalling it. Uninstalling alone does not delete the account.
First keep a copy of what matters: in the app’s settings, tap « Export » or « Share », then choose PDF.
Cancel the subscription, if there is one: Settings > your name > Subscriptions (iPhone), or Google Play > profile picture > Payments and subscriptions > Subscriptions (Android). Deleting the account does not always stop the payment.
In the app: Profile > Settings > Delete account. You should receive a confirmation email.
Then uninstall the app.
If you don’t see it: write to the publisher, at the address in its privacy policy, to request the deletion of your information.
Limitation: some data is kept for a period of time, for legal reasons. The privacy policy says how long. In Quebec, the law also lets you ask what information a company holds about you.
To picture it: uninstalling is closing the door. Deleting the account is emptying the house.