Look what happened when I renamed "Nagios" to "Monitoring Appliance":

{F148834}

That's the hierarchy for the projects page. Phabricator seems to be aware that Nagios doesn't exist, and renders it in italics. The child document, 'Sprints', is still there, but when I click it, I get a 404 response.

There is, so far as I can find, no reasonable way to get at or recover that orphaned Sprints.