This would be extremely nontrivial. It’s not just a mode you can toggle between. All of the shaders would have to be recalculated, which means you’d have to modify the editor to somehow build and retain both versions of the shader cache so you could distribute both. Beyond the technical hurdles in just getting that to work, you’d cause nightmares for your content creators. The performance impact of various pieces of content are very different depending on whether you’re using a forward or deferred renderer, and often what’s good for one is terrible for the other. Your content people would have to optimize for both, which basically means going with the lowest common denominator, and what fixes performance in one will often kill performance in the other. The forward renderer is still a work in progress, so it’s missing quite a few features, which means content would need to be built to avoid those features, or alternate content would have to be built and used dependent on the renderer that’s being used. Think of the bugs and testing time. Unless you have an extremely good reason that I can’t think of, it’s highly, highly unlikely that this is worth doing.