iOS 11 on the iPhone X removes the Reachability option for one-handed use16
Reachability is not supported on iPhone X pic.twitter.com/vvclAXMr2J— Guilherme Rambo (@_inside) September 13, 2017
Furthermore, as noted by Appleinsider, the feature also received a downgrade on devices other than the X: the Notification Center can't be accessed when using Reachability any more. Which is a pretty terrible decision, considering this is one of its major use cases.
But Rambo himself notes that, since this is a beta version of iOS, the feature may be implemented in some form when the X hits the shelves, while the Notification Center being inaccessible could be just a simple bug. Still, it seems pretty clear Apple isn't as afraid of small-handed people's wrath as it was back when the iPhone 6 debuted.