After years of waiting, Netflix finally added support for saving content for offline viewing last year. But until now, that was limited to the device's internal storage. The latest update, version 4.13, now supports downloading content to an external SD card.
To check where Netflix is saving offline content, go to 'App Settings' at the bottom of the hamburger menu, then tap 'Download Location.' At least on my tablet, SD card was set as the new default location.
The Netflix update does not appear to be a staged rollout, so it should be live for everyone on the Play Store already. If you prefer, the update is also available from APKMirror.