Answered: Tempering Shafts

According to <R5> “Robots may be built ONLY from Official Robot Components from the VEX Robotics Design System unless otherwise specifically”.

As shafts are official components, would they still be competition legal if they had been heat-treated/tempered to reduce plastic deformation under high torque?

I see nothing in <R15> that would make this illegal.

No, this is not legal. Metallurgic processes, such as heat treatment, fundamentally change the material properties of standard VEX metal parts. The result is a part which is no longer identical to a legal VEX part (per R7b). (Anodizing is the only process which is specifically allowed, as part of <R7d> part i)