Last I knew (several years ago) they were still required westbound on UP's Cima Hill between Las Vegas and Yermo if a train's tons per operative brake exceeded a certain amount. I recall watching a westbound unit coal train stop at Kelso so the crew could walk the train and turn down the retainers. Heat from the brake shoes was pouring off the train in waves. I remember the requirement was the same westbound on the SP between Azalea and Dunsmuir if you didn't have dynamics. Probably true a lot of other places as well.

There have been a number of times I've picked up a car at an industry or whatever and the air wouldn't release; a quick look at the retaining valve showed it to be in the holding position. No doubt a case of "gee, I wonder what this handle does..."