I’ve read the PCI 3.0 Specification and amdgpu Linux kernel driver. All it takes for Resizable BAR to be enabled is writing the new size to the PCI_EXT_CAP_ID_REBAR extended capability after reading the maximum supported size provided that there’s enough free virtual address space for it.
Linux amdgpu automatically does this and it works my system (though it has to move the BAR and bridge window before to make space). However the Windows AMD graphics driver doesn’t move/resize the BAR and expects the BIOS to do it which is why we see an option to enable it on new motherboards.
I’ve confirmed that Resizable BAR can work in Windows on Ivy Bridge which is shown on my screenshot. It was taken using a older commit of my program (GitHub - xCuri0/ReBarUEFI at 4e9175badbf7a4d75ef6ac4c4c92897dc559b7ae) which works by setting up the BAR and bridge window locations like how Linux does and then resizing it to the maximum size (8GB) before loading Windows bootmgfw.efi.