Could you please let me know how much the maximum voltage level of Source terminal pins of ADG5408 during power off ?
When you say "power off" do you mean VDD and VSS supply at 0 V?
If so the max ratings section of the datasheet states that the source terminal of the ADG5408 must be kept to within 0.3 V of the supplies.
Therefore if the VSS and VDD voltage is 0 V the source terminal must not exceed +/-0.3 V to avoid damage to the device.
If you require a higher voltage stand off, our fault protection and detection devices like the ADG5208F can withstand up to +/-55 V with both the power on and power off.
Let me know if this device would work for you or if not, let me know and we can work to find another solution.