While the feature has not returned in its most simplest form, the Xperia XZ3 does bring it back in a more convoluted way. If you delve into the “Ambient display” options and enable “Smart activation”, then double tapping the display will show the clock and notifications, double tapping again will show the lock screen. So the method is slightly contrived, but it works. However, unfortunately, double tap to sleep remains off the cards (for now at least).