With the changes brought by the Evolution of Combat regarding 2-hand and off-hand weapons, the Void Knight Deflector became less useful. The Void Knight set is most often used for its offence-boosting effect, but using a shield decreases damage output. Because of this, it is best used as an ordinary shield.
The deflector used to require 1250 Conquest rating to purchase, until the Minigame Spotlight update. Having a rank of 1250 is still one of the requirements needed for the trimmed Completionist cape, however.
Other void knight armour can be alched however the deflector cannot.