Izzet Delver is a fast tempo-aggro deck with Dragon's Rage Channeler and Murktide Regent, protected by Force of Will and Daze, so discard that can't take a threat is weaker here. Surgical Extraction can target a Lightning Bolt or other instant or sorcery in their graveyard and exile every copy from their graveyard, hand, and library, both slowing their clock and denying Murktide Regent's delve-based size. We cut Duress since it can only take noncreature, nonland cards and can never touch Dragon's Rage Channeler or Murktide Regent, keeping Thoughtseize for its ability to take any nonland card from their hand, including a threat, before they can protect it.