Using classic Outlook for Windows in business environments
This works in the old outlook but not in the "new" outlook. You need to add a reg key to allow copying again. I think in the new Outlook you have to use duplicate, but I haven't tested that.
Here is the reg key entry that has to be added for the old outlook
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Options\Calendar
EnableMeetingCopy=DWORD:1
After you add the reg key you can drag and drop from one calendar to another.