256c does not collect any user data, or tracks the user in any way. It doesn’t use cookies. It only stores the data the user inputted on the user’s device. This data can easily be exported from the Settings screen reachable from the Overview screen by swiping upwards after completing the initial setup.
However, 256c is hosted on Netlify in order to be publicly accessible. Netlify stores the IP addresses of site visitors for less than 30 days. For more information, please turn to their Privacy Policy and GDPR Compliance Statement.