On Pixel phones at least, holding down the power button will force it to power off without requiring any screen interaction. You have to hold it down for a fairly long time (something like 30 seconds).
If you've set up ``adb`` debugging, I believe you can also run the shell command ``reboot -p`` (not sure whether that requires root though). I.e., connect it to a paired computer, and run ``adb shell reboot -p`` from the computer.