I’ve had to do so recently due to some kind of transition error that gets the device stuck in between apps. On the iWatch, it’s just both button down for a long time. On the iPhone XS, it was stuck on the screen that you would use to navigate to different apps. I can’t leave this screen. Unlocking the screen shows me a deck of apps presently open but I cannot select any.
At first I rejoiced at being able to turn flash light on from right upper corner. However that would have taken a while for battery to run out. The sequence (volume up, volume down, hold power) doesn’t work, as it triggers Siri. So… the trick here is to try it on all the screen you can reach, including locking the phone and then trying to reboot from lock screen. On one of those screens, the above sequence worked, and I’m blogging about the experience now. This is circa mid 2019, iOS 12.1.4(16D57).