As Users who have created Calendars get deactivated, any Calendars which they have built and shared are no longer visible to those with whom they have been shared. This is inconsistent with how Reports/Dashboards are maintained when a User is deactivated.
There are 2 current workarounds:
Create a "fake" User account at the System Admin level and make a copy of the Calendar from the deactivated User, then re-share the new Calendar.
Log in as a current User and make a copy of the Calendar from the deactivated User, then re-share the new Calendar.
However, when making a Copy, it is important to note that if any Custom Date Fields on Calendars are used, those are no longer maintained. Therefore new Custom Date Field filters need to be built.
It would be nice to see that Calendars behave the same when a User is deactivated to maintain the Calendar Report and the Filters that have been applied.