As far as I know TAA can't be enabled in sli/crossfire due to it using the information from the previous frame to downscale the next frame, and many modern games are using TAA as the performance hit is usually positive. Gone are the crysis 3 days of choosing your AA method.
AC:Oddesy is a good example of this, fps will be higher on low or medium AA than with AA off due to the temporal nature of the TAA, and seeing as we are late into console generation any extra performance would presumably be a must for a dev.
I run a tech shop so have come into contact with a lot of enthusiasts over the years, and from my experience at least, after Pascal (1080ti specifically) the amount of dual gpu systems I see has significantly reduced, almost to the point of non existence. Even myself. I had usually started with one mid range card I could afford when younger, then added another. I had dual HD 5770, dual GTX 560, dual GTX 660, dual GTX 760, ended up upgrading them to dual 780 strix, dual 980ti classys, then single 1080ti, then single 2080ti. ( you could plot a graph of how successful I have been financially in my life by using my current rigs gpu
Then there's the sheer price of cards, I recently found a invoice for a client who purchased a 780ti off me in 2014, "Gigabyte GeForce GTX 780 Ti GHz Edition 3GB -- $599.00" That's AUD. The equivalent 2080ti today is $2249 AUD ( https://www.pccasegear.com/products/...gaming-oc-11gb
) That's a 275.5% increase from the top of the line GPU in 2014 to 2020. I would hazard a guess that dual GPU is completely priced out for more people then ever.
Due to all these factors I doubt its very profitable or even sensible for devs to support multi gpu, whether its via DX12 or otherwise.