@natanji Horde is known to reset VCard properties it does not know (see https://www.davdroid.com/tested-with/horde/#c111); maybe it shows the same behavior for events. Verbose logs would be needed to verify that.
The use case is to understand why adding an event to a calendar does not impact the freebusy calculations for the calendar owner. This means the user shall be able to see, that the busy status is ignored for events on the calendar.
Adding this in the property window is perfectly fine.