Face unlock is more widely available on smartphones nowadays, but many of us seem to forget that Android has always had a barebones — albeit easily fooled — equivalent of the feature for years. Android Smart Lock's Trusted face was added in 2014 and has been accessible to users on all Android devices until recently. Now, it's completely gone from stock and OEM devices, running Android 10 or below.
The feature was accessible under Settings -> Security -> Smart Lock -> Trusted face. It didn't use any biometric data for security, instead just relying on your face to unlock your device. A photo could easily fool it. The writing was on the wall for its removal: It was broken on Android Q Beta 6 and we know Google has been working on a more secure face authentication method.
Left: It's there. Right: It's gone.
But it's not only Android 10 that no longer has the Trusted face option. We've verified that the option is gone from the OnePlus 6T, Samsung Galaxy S9 and S10, Nokia 3.2, all of which are running Android Pie stable. That's because Smart Lock was never really part of the firmware, but was always controlled by Google Play Services — Trusted places, Trusted voice, and On-body detection were all added with a Play Services update.
So it's highly likely that a recent Play Services update removed the option. We don't know if it's just a temporary affair and it'd be back once bugs are ironed out or if it's gone for good.