Ability to Mark event as cancelled, postponed, did not take place
I wish to have the option to indicate a calendar event with statuses such as:
"Cancelled"
"Postponed"
"Did not take place"
or similar. This is intended to keep the event logged in the calendar (as it's not deleted) with a corresponding label. Furthermore, the event should be categorised as a "past" event, ensuring that no notifications or concerns arise on the scheduled day.
A "postponed" event could also be linked to a future event to indicate it was postponed to XYZ. There, it would be easier to automatically clone the main event to date XYZ+3 giving the user to chance to add further details.

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Fabian commented
The ability to cancel appointments without having to delete them would be really great: ideally, the calendar entries could take on additional properties or different color textures like hatched or semi-transparent to make them easily recognizable.
Currently, as a workaround, I move canceled appointments to an extra calendar.