it would be nice to have the option to turn off the flat lighting to mobs/entities/block entities

I don't know what you mean.

sorry for not clarifying, mobs and block entities still look flat when you turn off 'use flat lighting'

I'm afraid there's nothing I can do. 'Use flat lighting' simply enables an Optifine setting (and is only there for Iris users without access to it). If it doesn't include entities as well then that's a preference of the Optifine/Iris developers.