> I believe that Apple also exempts Radar, its internal bug reporting app used by Apple engineers.

Safari whitelists a number of URL schemes used by its first-party and internal apps:

  adir
  applenews
  applenewss
  itms
  itmss
  itunes
  itms-books
  itms-bookss
  ibooks
  macappstore
  macappstores
  rdar
  radr
  radar
  udoc
  ts
  st
  x-radar
  icloud-sharing
  help
  x-apple-helpbasic