We value your privacy at Ulysses. Be assured that your writing is treated with utmost confidentiality and security. Access to your data is strictly limited to you alone.
It's important to note that, in rare circumstances, your data may be accessible to Apple. This is because we utilize iCloud Drive for storing your files, and Apple has access to the encryption keys.
However, if you enable Advanced Data Protection, even Apple cannot access the data anymore! To enable it, please refer to this Apple Support guide: How to turn on Advanced Data Protection for iCloud
For the encryption to work, you need to store your groups and sheets in the iCloud section of your Ulysses' library.