What is the newest platform that lacks resizable BAR? It was standardized in 2006. Is 4060-level graphics performance useful in whatever old computer has that problem?
Also, I don't think you'll find many mainboards from 2006 supporting it. It may have been standardized in 2006, but a quick online search leads me to think that even on x86 mainboards it didn't become commonly available until at least 2020.
Coffee Lake also didn't really support ReBAR either, also 2018.
Another issue is that not every GPU actually supports ReBAR, I'm reasonably certain the Nvidia drivers turn it off for some titles, and pretty much the only vendor that reliably wants ReBAR on at all times is Intel Arc.
I also personally wouldn't say that Sandy Bridge is very usable with a modern GPU without also specifying what kind of CPU or GPU. Or context in how it's being used.
I cling onto my old hardware to limit ewaste where I can. I still gave up on my old sandybridge machine once it hit about a decade old. Not only would the CPU have trouble keeping up, its mostly only PCIe 2.0. A few had 3.0. You wouldn't get the full potential even out of the cheapest one of these intel cards. If you are putting a GPU in a system like that I can't imagine even buying new. Just get something used off ebay.
Consumer boards and CPUs didn't really support it well until after 2018. I upgraded away from a Zen+ system because it didn't support it.