This hook (specified in https://docs.moodle.org/dev/Calendar_API#Event_hook) is being removed in 3.3 by MDL-57578. This hook was added without much explanation. Developers should be using the Moodle events system to achieve this behaviour, not use this hacky calendar specific implementation. Events will be added in MDL-55611 for 3.3 which you can use if you wish to be notified of the hide/show calender event action.
General developer forum
Removal of calendar_event_hook()
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