Structured does not require its users to set up an account. Instead, all of your data is stored in your iCloud. If you want to remove the app data of Structured from your iCloud, you can do so in your device's settings by following the instructions on the Apple support page: Manage your iCloud storage.
As Structured takes your privacy very seriously, we do not collect any personalized data from you. The only type of data we store on our servers is anonymous analytics data that helps us to identify problems and improve the app. You can also stop sharing your analytics data with us at any time. Simply open the ⚙️ Structured Settings and select Analytics. Here you can deselect the option Share Anonymous Analytics.