The hygroscopic nature of hydraulic fluid is there for the same reason it is in car brake fluid-
it's better to evenly distribute water throughout the fluid, than to have it accumulate in a low point
it's better to evenly distribute water throughout the fluid, than to have it accumulate in a low point
Or if you leave a barrel of hydraulic fluid outside until it is partially water and it's not easy to notice when you are handling it.
But brakes can get really hot. Passenger brakes can easily get over the boiling point of water. Keep braking long enough and adding enough heat, and they'll get over the boiling point of brake fluid under pressure.
Thing is, brake fluid is incompressible. Brake fluid vapor however is very much so a compressible gas. Even more so: the water in the brake fluid has an even lower boiling point then that.
So stomping on the braking quickly isn't going to cause much of a problem. But if your riding the brakes down hill for a long distance with 12 year old brake fluid in hot Florida summer on very heavily loaded car? That... might get you into a spot of trouble.
I wonder if some YouTuber has done some experiment to see just how bad fluid had to get before a typical driver could notice.
It's not worth the expense to manufacture and it's not worth the risk. Not when a litre of DOT4 is $20 and that only needs replacing every 3 years at an aggressive schedule for passenger cars.
Larger vehicles you can purchase a brake fluid tester, but most of the really large ones I know use air brakes anyways.