Yeah Xamarin/Microsoft wanted to add some changes to the main UE4 version, but there are some legal. Xamarin has been talking about getting these legal cleared since September 2016. I has been 5-6 months and it look like it is still stuck :(. I’ll post a link of the post by de Icaza. I assume “integrated into the upstream UnrealEngine” meaning adding some changes from the Mono code to the mainstream UE4 version.