Sandy Bridge (2009) is still a very usable CPU with a modern GPU. In theory Sandy Bridge supported resizable BAR but in practice they didn't. I think the problem was BIOS's.
On paper any PCIe 2.0 motherboard can receive a BIOS update adding ReBAR support with 2.1, but reality is that you pretty much have to get a PCIe 3.0 motherboard to have any chance of having it or modding it in yourself.
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.
My old Ice Lake CPU was very much a bottleneck in lots of games in 2018 when I finally replaced it. It was a noticeable improvement across the board making the jump to a Zen+ CPU at the time, even with the same GPU.
Oh wow. That's older than I thought. This is definitely less of an issue than folks make out of it.
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.
There were a lot of generations after Sandy Bridge which didn't have it; Sandy Bridge was just one generation that didn't really support it on the consumer side.
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.
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.