Why does Archfiend of Ifnir mention cycling, if it already mentions discarding?
With the current card pool, and under current rules the 'cycle or' part is unnecessary. It is most likely there as a reminder so that players don't need to remember that cycling involves discarding (for the cards that don't have reminder text for it). (It is possible for it to matter, if for example something replaces the discard, but there are no cards that do that currently)

I am unable to find a quote at the moment but I know people at Wizards have stated that they don't always try to make the shortest rules text they can. They are willing to make it longer if it improves clarity.

And now we have the Amonkhet Release Notes which state (emphasis added):


  An ability that triggers whenever you "cycle or discard" a card triggers only once if you cycle a card. The ability "Whenever you discard a card" is functionally identical to this ability; cycling is mentioned for clarity.