Dear Shobi
A desuperheater can be considered a item of instrumentation but this depends on which system you use. Lets get practical: there are control valves that are specifically designed to desuperheat steam (like Samson type 240 or pressure and desuperheater type 281), this are clearly part of instrumentation. There are receivers or spray nozzles system used for this too and, even its use to control the temperature, I would not classify this as an Instrument.
Kind regards