How to use "unavailable" as a trigger/status

alias: 'Notify if light unavailable'
initial_state: 'off'
trigger:
  - platform: state
    entity_id: light.bedroom_lamp, light.couch_light, light.dining_table_light, light.living_room_light, light.walkin_robe_light
    to: 'unavailable'
    for:
      seconds: 22
action:
  - service: notify.pushbullet
    data_template:
      message: >
        '{{ trigger.from_state.attributes.friendly_name }} has gone unavailable'

https://community.home-assistant.io/t/how-to-use-unavailable-as-a-trigger-status/205388/30?u=msly

http://localhost:4999/boards/topic/22984/%E5%AE%9A%E6%97%B6%E9%87%8D%E5%90%AF%E9%9B%86%E6%88%90-reload-integration