From xiyuan@:

KioskExternalUpdater is added for  http://crbug.com/372857  to allow manual kiosk update via usb stick for deployment that has no network access. Do we know whether we have real customers for the feature? We should consider removing the feature if there are no real use cases.

If we need to support it, KioskExternalUpdater works in a funny way that it is started when the device runs any kiosk apps and updates the cached CRXs for kiosk apps installed on the device and provided in the usb stick.

From me:

Basically, we need to explore usage of the feature, e.g.via UMA metrics.
After that PM will create a plan of deprecation if needed.