I’m sure a Skylight with a flat colour, or maybe a Y gradient so there’s no light below the horizon, would work. Then bake it down. Essentially it’s an Ambient Occlusion or you can tweak it to not do the shadowing.
Looks like there’s a directional shadow - so you could balance the two lights to make it flat.
Another point is that only furniture or static geo is casting a shadow and not the characters/dynamic geo - you have the ability to do that too.
Matt Hermans