The clue is in what you said - Forward Renderer. That’s a fundamentally different approach to rendering. MSAA is more expensive for deferred as outlined in the PDF which for some reason people are allergic to reading through.
Please read the PDF, or even just glance at the screenshots I took the time to add. I don’t really know how to make this information more accessible.
I’m no graphics programming expert, I’m just presenting the (dated) information given to us directly from the horses mouth - AKA one of Epic’s senior rendering engineers. That PDF of slides explains in less than a couple of sentences why MSAA is apparently not a valid option for (specifically) UE4’s deferred rendering pipeline.
According to the PDF, MSAA requires shading an individual pixel more than once. You don’t need to be a graphics programmer to connect the dots, and realise how quickly that becomes a problem even with low amounts of MSAA. I’m sure there’s some obscure GDC presentation about some other technique some studio did that fits their specific use case perfectly - but that doesn’t always apply.