Noteworthy that the only card of the three you can use in mGPU is the 1080ti. 1080ti SLI pi$$es all over all alternatives where supported for a far better price. Not surprised mGPU is no longer supported, it's the best way to get decent fps at a decent price hence the duopoly won't allow it. AMD/Nvidia (in no particular order) are having a laugh by stopping supporting the best value option in the GPU space. I'm running RDR2 all ultra except MSAA at 2560/1440 at 79fps. That's with 2 cards from March 2017! Both of mine steaming ahead at 1962/1949 core clock consistently in games.
Conspiracy Theory nonsense. Turing supports SLI just fine https://www.youtube.com/watch?v=cbvFb0IHTg0
SLI/xfire simply does';t work on many modern rendering engines, and with DX12 SLI support is up to the developers.
It is simple economics. Almost no one owns an SLI setup so no one is willing to invest in it.