Starting with version 0.94 known_devices.yaml is being phased out:
Modernizing the device tracker
This release also introduces a long overdue overhaul of how the device tracker works. We are introducing this overhaul piece by piece, focusing first on device tracker platforms that push their updates to Home Assistant: mobile app, OwnTracks, GeoFency, GPSLogger and Locative.
These integrations will no longer use known_devices.yaml but instead use entities, like all other integrations in Home Assistant. You can change the name and entity ID via the UI. It is no longer posible to merge the devices with other device tracker entities, this includes using the see service. This was flaky at best. You should now use the new person integration for this.