Those familiar with digital art and photo processing may know about the “erase” blending mode. In this mode, any opacity would erase the content of the layer below. If added to GDevelop and made to interact with only certain layers (or made to interact only with layer directly below it), the game creator could make a shadow/light effect. GD already has normal, add, multiply and screen modes. I don’t know how to put it into words efficiently so there’s a doodle explaining it below:
(Note: I don’t know of any other way of doing this as efficiently nor how hard it would be to implement it in GD’s code, but it would be useful for things like multiple sources of lighting and transitions in and out of scenes, Super Mario style, or other things)