If I use Hidden Blade on a unit but the unit doesn't die because of an effect, does my opponent draw or not?

Ruling: Whether your opponent draws depends on what type of effect prevented the death. If the unit was protected by a replacement effect (like Zhonyas or Sett legend), your opponent still draws. If the unit was returned to hand or otherwise removed, your opponent does not draw.

Nuances:

• Replacement effects (like Zhonyas or Sett legend) keep the target available for resolution, so the draw still happens
• If the unit gets bounced or removed, there is no last known information to identify the controller, so the draw does not happen