With the iPhone 8 and iPhone 8 Plus, you need to follow three steps in order to do a force or hard reset. The process involves:
- Press and quickly release the Volume Up button.
- Press and quickly release the Volume Down button.
- Press and hold the Side button (power button) until you see the Apple logo.
To power off the phone, hold down the Side (or power) button for a few seconds until the “slide to power off” slider bar appears. Added to iOS 11 this year in the Settings app is a “Shut Down” option that can also be used to turn off the phone.
source: AppleSupport via MacRumors