Usually, zigbee2mqtt will provide an availability topic for each device that it controls.
zigbee2mqtt/yourDeviceFriendlyName/availability
When the device comes online, zigbee2mqtt will publish online to this topic. When the device goes offline (such as power has been removed) zigbee2mqtt will publish offline to the topic.
So you get instantaneous notification whether it’s online or offline - no need for rules! See this simple example for a dimming bulb.
(You will need a rule to send a notification, of course, but it’ll be very small and simple!)