The healthStatus attribute is used by some of the community drivers. HE stock drivers do not support it. But, it is actually useful if you have some more complex RM5 or WebCoRE rules, where the logic flow can be split depending on whether the device is offline or online to the hub.
The Device Activity Check app also works with the 'Last Activity' property of each device, and this is a much more universal approach. So the 'device is offline' notifications should be fired after the pre-configured hours without receiving anything from the device.
