Is my data secure (encrypted)?

NotePlan doesn't encrypt your notes but saves them locally on your device. CloudKit or iCloud Drive are used to synchronize your notes across your devices. Both are services maintained and encrypted by Apple. So, during the sync process (upload & download), and while being stored in the cloud, your data is encrypted as per Apple documentation.

As the developer, we have no access to your data. Your data can be accessed only with your private Apple ID.

Additionally, to secure the locally saved data, you can enable "FileVault" in you system preferences: