I have found many recipes for legendary helms that are part of a set such as The Clairvoyant, Ultos’ Storm, etc. And I have dozens of non-set legendary items. But I have never seen either a drop or a recipe for the remaining pieces of the sets that these crafted helms go with. Which is it…item drop or recipe drop? Either way, they must be incredibly rare.
As far as I know only head pieces from sets are craftable rest is item drops.
All legendary helmets with the exception of Outcast’s Secret can only drop as blueprints. There are other legendary blueprints for other slots but none are for sets.