Well I think it was at least before post production, I remember an interview with Mark where he said George pulled him aside before shooting the scene and told him so he would know to react stronger than expected.
Kershner pulled him aside just before they shot the scene, and told Hamill that George knows this, I know this, and now you know this. the implication was that if this leaks, it will have come from YOU.
On the set, David Prowse said ‘Obi Wan killed your father’, so the crew and other actors didn’t know the truth.
Even Harrison Ford only found out at the screening.
It was one of the best kept secrets ever, I can’t imagine them pulling it off now.