core/homeassistant/components/notify/icons.json

{
  "entity_component": {
    "_": {
      "default": "mdi:message"
    }
  },
  "services": {
    "notify": {
      "service": "mdi:bell-ring"
    },
    "persistent_notification": {
      "service": "mdi:bell-badge"
    },
    "send_message": {
      "service": "mdi:message-arrow-right"
    }
  }
}