My online/offline status driver that @Paul_Oliver referenced could be used if having my edgebridge app running on a LAN computer is a possibility.
It could be used in an automation or Rule that does the notification if offline for a period of time, rather than getting notified immediately. One could utilize my Edge Counter Utility to trigger after a period of time similar to this use case.
Or, as pointed out, you could do a custom script that would run on a LAN computer (either using CLI or API). The downsides to all these solutions is the need to poll the device(s) for their online/offline statuses. A SmartApp is really needed to be able to subscribe to states and get asynchronously notified.