My wife sends some invitations from her proton calendar to her work calendar which is based on exchange. She always has to edit these events in work calendar afterwards and set the "private" flag, so that the the details are not visible to colleagues.
We would need a private flag for events in calendar that can be set, so that it is recognized from exchange and other systems that the invitation/events hast to be set to private state and thus details are not visible in the foreign system. Setting this flag should also work via mobile app.