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Last tested: Mar 25, 2019
The custodian was removed in Looker v6.6, so only older versions of Looker will show this message.
The custodian ran during the Looker start up process and cleaned up any schedules that haven't completed and have no chance of completing. An example of this is a schedule that is in the "execute" state but there is no query running on the database for this schedule.
So that error message indicated that the schedule was cleaned up by the custodian because there was no change of completion.