That sort of exists, but only sort of. If you press the lock button on the side of the iPhone five consecutive times, it will then require your passcode to unlock (hopefully a high entropy passphrase), and will disable biometric authentication until unlocked with a passcode. You can set the phone to wipe after 10 failed attempts to unlock.
You can also say "Hey Siri, whose phone is this?" and your phone will lock down the same way as described above.
Of course, this doesn't protect from the $5 wrench attack, but plausible deniability only goes so far as well in a targeted attack. At least, depending on your local laws, law enforcement may not be able to compel you to provide your passphrase, but they can easily force you to use your biometric data, so this protects against that.
You can also say "Hey Siri, whose phone is this?" and your phone will lock down the same way as described above.
Of course, this doesn't protect from the $5 wrench attack, but plausible deniability only goes so far as well in a targeted attack. At least, depending on your local laws, law enforcement may not be able to compel you to provide your passphrase, but they can easily force you to use your biometric data, so this protects against that.