http://wccftech.com/power-down-iphone-x-how-to/

Here’s how you can power down the iPhone X. No, just pressing the power button doesn’t turn it off.

Since the first iPhone all the way to the iPhone 8 and iPhone 8 Plus, the process of powering down the device is pretty straightforward. You hold down the power button for a couple of seconds and you’ll be greeted with the Slide to power off screen. Just slide the slider and the shut down process will begin.