My bush hog has a pressure relief vent plug that looks just like this one. Mine has a small ball bearing that's held closed with a small spring. I like it because no matter how much water I go through I know it's not going to get in the gearbox.
I was just noting that my tractor does have the vent. I think many tractors are vented, it's just hard to see the vent. No one in particular...Sorry, but who's tractor are you referring to?
(Mine did not have a vent for sure, but has one now)