Hold the lock button and one of the volume buttons for 3 seconds. This disables biometrics, and gives on-screen swipe options for MedicalID and Emergency SOS. Pressing the lock button 5 times also disables biometrics, begins a 3-second countdown to auto-call your Emergency SOS contacts and 911. One nice touch is that the language used to let you know Touch/FaceID are disabled doesn’t rat you out to the cops (just says that Touch/FaceID doesn’t recognize you, not that you intentionally disabled it). If these don’t work for you, there are options under Settings, Emergency SOS.
Wow. Just tried this on my 6S (iOS 11). When asked from the lock screen, I was required to enter my passcode to unlock afterward.
Just did this with my XS. It immediately disabled face unlock and required password. Neat.
Just tried on iPhone X. It didn't disable FaceID.
Contrary, from the lock screen it disabled biometrics. Your phone must be locked already.