Your data will be hosted on the Atlassian-providing infrastructure called Forge. Even the app developers don't have access to any stored data unless you explicitly provide the app log. For reference, visit https://developer.atlassian.com/platform/forge/access-production-logs/ to know about the app log.
To comply with GDPR, this app will delete the stored credential data within 7 days once your Atlassian account has been deleted, and the backup automation will stop working.