ArcGIS Monitor can detect when component is not responsive. This could be a case when it is "down", but also when user shuts down a component for maintenance: a host, arcgis server or portal.
In the current release, user can be notified through notification, as below. In the next release, there will be additional metrics where users can set alerts. Also, we'll be changing status names to be more descriptive, e.g. instead of "Error", "Not reachable"