Yes a line isolation contactor or shunt trip circuit breaker is needed with Schneider Electric ATS48 Soft Starters.
Since the solid-state switches may be incapable of completely blocking the motor power should a fault occur, auxiliary isolation on the line side of the controller is required. Use either a circuit breaker equipped with a shunt trip coil or an electromagnetic contactor. Connect the isolation device to the fault relay of the controller so that it opens the controller power circuit in the event of a controller fault. The isolation device must be capable of interrupting motor locked rotor current.
See link for further product information:https://www.se.com/au/en/product-range/689-altistart-48/