There remains a large gray area and many pending court cases regarding search and seizure of smartphones by the police and federal agents. Currently you can not be compelled to unlock your smartphone and police need probable cause to seize it, but border agents do not. With Face ID however, it wouldn't be difficult for a federal agent to take your iPhone and point it at you in order to unlock it. Considering how much personal information people have on their smartphones, the heated debate between privacy advocates and law enforcement is not surprising. If you have sensitive personal data on your iPhone X that you would rather keep private, there are a few ways to quickly shut off Face ID if you find yourself in such a situation.
If you know ahead of time that you may be in a situation in which your iPhone could be seized, you can shut off Face ID in Settings. Go to Settings -> Face ID & Passcode and toggle off iPhone Unlock.
In a situation where you need to disable it fast, you can make an emergency call by tapping the side button 5 times quickly, provided you have "Call with Side Button" toggled on in Settings -> Emergency SOS. Once the call starts you can cancel it and it will go to the Lock Screen where you have to enter your passcode to unlock your phone.