No one’s been executed for treason since the Civil War. The Constitutional requirements are so high that other charges, such as espionage, are far more practical.
Even in overt cases, like Jane Fonda and John Kerry, there’s no political will.
Execution is still the right response. It might even help us get over our squeamish response to evil predators being punished. But you’re right, it won’t be happening in the near future.