A 400 V delta / 698 V star motor can safely run continuously at 400 V with the star connection. However the motor should not be operated continuously stalled or with high slip. Operating stalled would mean that most of the input power would be dissipated in the rotor and the rotor would not be cooled as effectively at stall as it would be at normal operating speed. The same would be true to a lesser extent for operation at high slip.
The posted curves indicate that the motor is a high-slip design motor. As the load is increased, the slip increases and speed decreases fairly smoothly. That provides some protection against the motor suddenly "breaking down" and going rapidly from a fairly high speed to stall.
It seems like there would not be a good reason for operating a motor in this manner.