Can PS-5S Electrode Holders be used to detect fluid level in warm (70 degree C) water?

No, PS-5S cannot be used for this. Steam would cause condensation to form inside PS-5S and result in insulation failure between its Electrodes.
In addition, the operating temperature range of the PS-5S is -10 to 70 degree C.
Depending on the amount of steam present, use either BF-5 or BF-1 Electrode Holders and allow some space between each Electrode.

BF-5 or BF-1 Electrode Holders can only be used, however, when there is no pressure applied. In other words, the operating pressure must be atmospheric pressure.