Under Withering Siphon prestige, Dark templar’s void stasis does not work on heroic units, gives the “cannot be used on heroic units” error message.
Shadow guard calldown’s void stasis does work fine on heroic units.
It still not work today.
I have this problem too: I am using Vorazon Prestige 2, Withering Siphon on Co-Op. On the Oblivion Express Map, I can cast Void Stasis on the trains iff I’m using Shadow Guard. Normal Dark Templar cannot cast Void Stasis on the trains, giving the error message that it cannot target heroic units. However, the prestige description explicitly says that Void Stasis is supposed to be able to target heroic units.