What is the minimum PWM frequency that LT1999-10 can capture with smooth output settling?
The plot of \$V_{OUT}\$ in your question is, in effect, the plot of motor current when driven with a square wave voltage of 100 kHz and is therefore triangular in nature. This is because the motor is basically an inductive device and if you apply a high-speed square wave voltage to an inductor, you get a triangle wave of current.

since plot look like degrading in output voltage until next pulse
appear. will it got to 0V before next pulse appear when pwm is 100Hz?

No, you are mistaking normal current waveforms for some degradation in the processing of that signal. This won't happen. Just look at the circuit inside the red box: -

Clearly, the LT1999 is monitoring current in the motor and the waveforms show no degradation.