As defined in https://tools.ietf.org/html/rfc6638#section-9.1 .
Display an icon for transparent calendars
Do you mean that the DAVx⁵ calendar list shold indicate whether a calendar has this property? This could be useful, but can you please give more information, for instance, a usecase and why this is important? I would like to show only “important” information in the overview, otherwise there would be dozens of icons for every calendar. Maybe it can be shown in the Properties dialog (which should be extended anyway)?
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.