There is a new downtime report in the reports page (Admin >> Reports >> Other Reports) in the 'Other Available Reports' section. It summarizes hours for maintenance users versus hours for non-maintenance users and calculate the percentage of downtime. This report can be run for a single resource or all. And, it can be run for a given year or the previous 12 months. An example is displayed below.
Note: Other hours includes superseded schedules since these schedules have not been cancelled and are still valid schedules. For a more accurate picture, superseded schedules should be cancelled.