The Ghost Experience Update Trailer showcased two features: drone vertical fall + reduced crash noise, as seen here https://www.youtube.com/watch?v=epqQYjBvf3k&t=43s
Has anyone seen those work? I probably (stealthily) destroyed more than 50 flying drones since this update, and not a single one fell vertically. Also, I didn't notice any reduced crash noise, as any enemies within a pretty large radius are still immediately alerted. Overall, I still heavily sigh anytime I see any of those drone on a mission, as I know there's not much I can do to complete the mission stealthily.
The example of vertical fall in the video is from a very small height and against a stationary target, making it look like the drone is falling straight down. In actual gameplay, the falling trajectory is determined by the speed and direction the drone is travelling.
When you destroyed a flying drone before the update, the wreckage could end up on the other side of the base and explode loudly, which would instantly alert every enemy in the base and the surrounding areas. After the update, the wreckage should drop somewhere in the immediate area and the explosion is far less audible.
The best way to eliminate flying drones, is to use the Echelon class' shock pistol. It will destroy the drone and nearby enemies will only enter the yellow alert phase. I recommend being within 100m of the target, to ensure the drone's destruction. Any further than that, you risk only damaging the drone and making it mad.