My iPhone 8 Plus is locked while playing Wordscapes. How to shut down?

See the following from If your iPhone won't turn on or is frozen - Apple Support

If your screen is black or frozen

Follow the steps for your device.

On your iPhone 8 or later, including iPhone SE (2nd and 3rd generation)

1. Press and quickly release the volume up button.
2. Press and quickly release the volume down button.
3. Press and hold the side button until you see the Apple logo.
4. 
5. 
   
6. If your phone doesn't turn on, follow these steps to check your hardware and charge your phone. You might need to charge for up to an hour.
7. If your phone still doesn't turn on, contact Apple Support.