Shadows
Shadow is a long-standing feature of FlightGear. The old shadow feature became deprecated as of 1.9.0 and it was re-implemented during the shift to OSG.
At some point scenery object shadows was reintroduced, probably when scenery objects like for example vegetations and buildings became rendered instantiated rather then as separate objects.
With FlightGear 2024.1 (link to changelog) aircraft shadows was reintroduced.
Shadow mapping
Shadow mapping is a technique that use a texture where distance of objects from the light has been recorded. During rendering, the distance to the light of the fragment rendered is compared to the shortest distance of the scene to the light, and if it is higher, the fragment is drawn using only ambient light. Otherwise, it is fully lit.
Project Rembrandt was a new project that implemented shadow mapping for FlightGear.
Another project which allowed aircraft developers to implement aircraft shadows was the Atmospheric Light Scattering (ALS) project.