Interesting feature huh? Right now the skies would be quite empty though. But yea, next to ading real-time weather Microsoft Flight Simulator will use real flight information to simulate real-time air traffic.
This is confirmed by Sebastian Wloch, director of Asobo, in the latest video in the series of new features. The flight information will simulate "most, if not all" of the planes that fly over the world at any time, and when the game is offline, an AI will take over piloting them. Meanwhile, if players activate Live Players mode, the weather will also be simulated based on real-world information. For example, if you fly over Paris in the middle of a storm, that's because there is indeed a storm in Paris.
Live Players mode will be the main simulation of the game, but there is also the All Players mode, which allows you to control practically everything in the simulation, including weather. The latter will be more attractive for those who only want to fly and enjoy the landscapes of the world without paying much attention to reality.
Update April 22nd: the specs have been released as to what is recommended PC wise. Obviously you'll need a beefy PC. Flight Simulator requires at least 150GB of storage. Just 8GB of ram and an AMD Ryzen 3 1200 or Intel Core i5-4460 processor at a minimum, with 2GB of VRAM from a graphics card -- with Microsoft requiring an AMD Radeon RX 570 or NVIDIA GeForce GTX 770 graphics card.
If you really like some eye-candy, you'll need a Ryzen 5 1500X or Core i5-8400 with 16GB of system RAM paired with a Radeon RX 590 or GeForce GTX 970.
The "ideal spec" is a Ryzen 7 Pro 2700X or Core i7-9800X / 32GB of system RAM and a something like a Radeon VII or GeForce RTX 2080 graphics card. Interesting that a Radeon VII is listed, likely in the best graphics conditions the simulator will eat up lots of VRAM.