I think you mean "hygrometer" i.e. device to measure humidity, not "humidistat" i.e. device to adjust humidity in the room to remain constant
There are indeed some BYCers who have said they have hygrometers in the coop. (To some extent I'm probably to blame for that, having pointed out that it's an option if you want more than seat-of-your-pants data to manage by
)
But two things to remember: 1) when you buy a hygrometer from the store it will hardly ever be accurate (and even if it is, may not STAY that way), so you need to use the salt method (described elsewhere on this forum) to calculate a correction factor to let you translate what the dial says into real r.h.; and 2) I don't know enough about it to guarantee that hygrometers intended for room temperature use are accurate when used at freezing or below. (I really *don't* know)
If you do want to install a hygrometer, you are shooting for ideally around 50-60 % relative humdity, certainly below 75ish %.
But IMO there is a lot to be said for just learning to eyeball it -- when you first go into the coop at dawn, is there condensation on surfaces other than windows, is there a LOT of condensation on windows despite not too much temp difference inside vs out, does the air feel damp.
Good luck, have fun,
Pat