You can completely uninstall Standard Notes off your macOS, Windows, or Linux machine by following these steps:
Open the application, then press Help in the top level menu bar, then Open Data Directory. Delete all files inside this directory.
In the same top level menu bar, select Backups, then Open Backups Location. Delete all backups.
Finally, close the application, then delete the application file for macOS, the AppImage file for Linux, and run the uninstaller for Windows.