Problem with Bode diagram in LTspice
The AC analysis is a linearized analysis. That means it simulates the small-signal behavior of each element at its operating point.
In the case of the switch, that means it is held in either the on or off state, and behaves as a resistor with either the ROFF or RON value.
The -96 dB transfer gain is due to the leakage through the switch's 1 megohm off resistance.
If the reason you're simulating this circuit is enable designing the switching control loop, really you should just remove the switch and diode and drive the left end of the inductor with your voltage source. Because the control loop critical frequencies will be much lower than the switching frequency, the whole switching circuit (input source, switch and diode) just looks like a voltage source with a value depending on the switch duty cycle when you're designing the control loop.