Socket extensions do NOT affect the accuracy of a torque wrench.
Every action has an equal and opposite reaction; newtons third law. If you turn one side of the bar, the other end must have an equal affect on the other end. A bar under torsion (twist) acts like a spring. The stored energy in the ‘spring’ (torsion-ed bar) is a large force applied in a short period of time, and can be balanced by a smaller force at a longer period of time.
The longer the extension, or the thinner it is, the lower the spring constant. The spring constant is lower because a longer or thinner extension is less stiff and is therefore more springy.
In the real world, when talking about extensions bars and their extremely high spring constants, that delay between applying force on one end and it being fully applied at the other is in a matter of milliseconds. For those paranoid, once the force is applied, hold it for one second and you will have given the bar many magnitudes more time than it needs.
In laymen terms, imagine a car and its 4 springs on each corner; regardless of how strong or weak the cars springs are, the full weight of the car will always be transferred to the ground when stationary. This is because the force at the top and the bottom of the spring are the same, given time. When you drive, the springs delay the force between each end of the spring because the force of a bump is only momentary.
This video illustrates that even with 100 inches of extensions (absolutely ridiculous length), the torque accuracy does not change.
As does this video:
This does not apply to torque sticks used on impact wrenches. The hammer on an impact wrench does not hold pressure, it only hits momentarily.