WTH cannot define icon for binary_sensor template!?

Month of "What the heck?!"
1

Why can you not specifiy an icon for a binary_sensor template DESPITE the documentation saying you can?

binary_sensor:
  - platform: template
    sensors:
      tesla_arrived:
        friendly_name: "Tesla Arrived"
        icon: "mdi:car"
        template: "{{ as_timestamp(now()) - as_timestamp((states.device_tracker.ryan_s_tesla.last_changed) <= 300 and is_state('device_tracker.tesla', 'home') }}"

Errors:

Invalid config for [binary_sensor.template]: [icon] is an invalid option for [binary_sensor.template]. Check: binary_sensor.template->sensors->tesla_arrived->icon. (See ?, line ?).
2

This may be due to using the legacy format…

3

Because in legacy format you need to use “icon_template”.