iPhone X and iPhone 8 reassign the double press and hold to Emergency SOS, since it's a simpler, easier button combination that'll better hold up to the stress of emergency situations. But that means moving reboot to a new and more complicated button combo.
Quickly click and release the Volume Up button.
Quickly click and release the Volume Down button.
Press and hold the Side button (sleep/wake, on/off).
Keep holding down the Side button, even after the Shut Down screen shows up. (iOS 11.2 and later.)