If we have lock-out/tag-out system and it is clear the disconnect switch will not be operated when it is powered, do you think we still need to put Arc flash hazard label on disconnect switch?

I think that the easy answer is yes.

The more complex answer may be why not? If it is going to be operated while energized and not under load, it will still need a label. There is a chance of fault while it is operating, and it may still be opened while energized which will present a hazard.

If the switch can not physically be operated while energize, what is its purpose?

Thanks for the answer. What if the switch is only acting as local disconnect means? When you have a TOLO program, you will not have a condition to operate this switch when the circuit is energized.

NFPA 70E article 130.2, and 130.3 state that an arc flash analysis has to be done to set the approach boundaries. This information is required to be posted on every electrical panel in the form of a label.