Tom's Hardware offers some background details over here. At the moment, Resizable BAR works exclusively in tandem with AMD's Radeon RX 6000 series GPUs but NVIDIA has already confirmed it's coming to its Ampere-based GPUs in the future. NVIDIA did not provide an exact timeframe though.
We're not surprised that motherboard vendors are already rolling out Resizable BAR on Intel platforms since the technology is part of the PCIe specification after all. Shocking as it may sound, AMD's engineers are working with both Intel and Nvidia to get the feature working on the latter two's products. It's touching to see companies putting their rivalries and pride aside for the benefit of consumers.
AMD Smart Accesss Memory Support jetzt auch auf ASUS/ROG-Mainboards mit Intel Z490/H470/B460-Chipsatz. Ab Beta-BIOS 1002 bzw. 1601 pic.twitter.com/dOVf3C41rh— Chris Wefers (@chris_wefers) December 1, 2020