I’d like to be informed if I need to look at a device that became unavailable because e.q. the battery is empty or device itself got broken.
So there should be a trigger if lets assume device D1 is unavailable for at least 24 hours so I coudl inform myself by telegram, mail or whatever 
Haven’t found an easy way yet to realize this.
Really easy to do
- trigger: state
entity_id: sensor.abc
to: unavailable
for: '24:00:00'
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The problem is that there is really no way of knowing when a device is overdue in reporting in.
Some devices report in every hour, some every 6 hours and some each 24 hours and everything else in between and outside these intervals.
It is not certain that a report signal is received, so you might have to wait 2-3 times the device report interval to be sure it is actually unavailable.
Ummm… just send yourself a notification if state goes to unavailable…?
It is not that easy.
A device that is offline might not be detected for quite a while and maybe not ever.
Zigbee and Z-wave have timeout setting for battery powered devices and one for mains powered, but it is still not perfect.
Some devices might have timeouts that are several times higher than the ones with the lowest timeout.
If you have not set the timeout, thwn it will never go unavailable.
Zwave doesn’t have timeouts for items powered by mains, it knows immediately if a device goes unavailable.
There will always be timeouts, but it might be low.
If the device lose power, then it will have no way to tell that it is offline, so a timeout at the controlling device is the only way to do it.
Ethernet wired devices have a link detection feature, but the same feature in wireless setup would require a constant carrier signal, which would prevent other devices from using that frequency, so it is simply not feasible.