The information I received was quite technical so I’ve tried to summarise it in to something easier to understand!:
Before, Auric missions were slightly easier but had extra modifiers to make them harder. When Auric became a regular Difficulty, that tougher version became the new standard. Then a new, even harder tier was then added for certain missions marked as HI-INS.
So, today’s regular Auric missions are about as hard as the old special Auric missions used to be.
Timed hordes appear only on missions marked as HI-INS or Auric Maelstrom.