core/homeassistant/components/persistent_notification/icons.json

{
  "services": {
    "create": {
      "service": "mdi:message-badge"
    },
    "dismiss": {
      "service": "mdi:bell-off"
    },
    "dismiss_all": {
      "service": "mdi:notification-clear-all"
    }
  }
}