LSF0108: We want to check whether the A side and B side is high impedance condition when not give the VCCA or VCCB.

The LSF does not contain logic, and it does not have supply voltages.

The level shifting is done with plain MOSFETs, and Vref_A/Vref_B allow some current to flow through the reference transistor in order to control the gate voltage:

When Vref_A or Vref_B is floating, there might still be a charge in the gates.

To force the I/O pins into high impedance, you must pull the EN pin low (or force both Vref_A and Vref_B to zero). See the training video Using the Enable Pin with the LSF Family.