Configure private file path in settings.php file

With help from others in the Backdrop zulip chat, the private file path can be configured in the settings.php or settings.local.php using either relative or absolute paths. To do this, add the following line:

$config['system.core']['file_private_path'] = '../files_private';

The private file path will show in the GUI at admin/config/media/file-system and it leaves the default download method as public, however modules that require private files such as backup_migrate and the demo module for snapshots will use the path. The default download method can be changed to private as needed.

Subsequently, changing the private file path in the GUI will likely not work as it will be overridden by the settings.php or settings.local.php configuration. Nonetheless, there are cases where this approach may be useful.