The new Flight Simulator is an upgraded version of the engine that dates back to at least 1990s, if not earlier, and traces of the fact are obvious. Legacy aerodynamics simulation from Flight Simulator X (2006) is included and fully functional. Air traffic control system introduced in Flight Simulator 2002 is also there with minor changes throughout the years. Even some artwork is 20+ years old, such as "737-400_scenery_t1.bmp" texture map[1] for a fictional 737-400 that came with Flight Simulator 2000[2]. I guess they forgot to remove it, or left it for backwards compatibility reasons.

[1] https://imgur.com/a/KnP1NKl

[2] https://microsoft-flight-simulator.fandom.com/wiki/Orbit_Air...