Swiping to change brightness still works on iOS 8. I'm using an iPhone 5.
However, it has long been the case that this functionality is finicky: you have to swipe precisely in the middle of the screen, with sufficient vertical movement to have the app register the action. Swiping in the way you might scroll down a page—with your thumb off-centre and only making a small down- or upward movement—doesn't work.
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