You can download the logs or configure a webhook to maintain a backup at your end.
Related documentation link:
As Netcore email API deals with your business-critical transactional emails, so maintaining the top-notch security and privacy for your data is our utmost priority. Because of the same reason we have added a policy to the email activity logs for the last 90 days only.
In case your business demands to store the data for more period of time, then in that case it is always recommended to give backing up the data at your end.