Soft-start refers to the device fitted inside the control panel to reduce the voltage applied to the motor terminals during start up. The term has nothing to do with the motor itself.

Soft-start units are available off-the-shelf and do not rate as a clean-sheet design project.

The most common use of the term soft start for a motor is an external soft start circuit. But the different induction motor rotor design categories will make an motor with a soft start like response. One pays for this in torque versus speed.