Surprisingly, you can get the current macOS Catalina running on 2008 Mac(Book)s with the tools published by dosdude1 and other contributors [1].
macOS Catalina will actually still get security updates for two years after Big Sur is released (they support macOS releases three years after release). So that's until ~late 2022, a pretty long run for that 2008 hardware until you have to go to Linux or Windows for your patches.
It looks like Big Sur has a hard requirement on Metal (at the moment at least), which kills all pre 2012 Macs [2] that were still running Mojave/Catalina using OpenGL.
[2]: Metal is supported on GPUs since Intel Ivy Bridge and Vulkan compatible AMD/Nvidia hardware. The Mac Pro 2010 is the exception since you can upgrade its GPU to a Metal compatible one.
>The Mac Pro 2010 is the exception since you can upgrade its GPU to a Metal compatible one.
You can also do this with the 2008 Mac Pro; either by using a recent nVidia GPU (though every time I've done this I've seen a bunch of weird artifacting) or by using an AMD GPU with the patch that removes the requirement for the SSE 4.2 instruction set (which the CPUs in the 3,1 don't support).
RX570s and RX580s being 100 dollars makes the decision upgrade that much easier.
macOS Catalina will actually still get security updates for two years after Big Sur is released (they support macOS releases three years after release). So that's until ~late 2022, a pretty long run for that 2008 hardware until you have to go to Linux or Windows for your patches.
It looks like Big Sur has a hard requirement on Metal (at the moment at least), which kills all pre 2012 Macs [2] that were still running Mojave/Catalina using OpenGL.
[1]: http://dosdude1.com/catalina/ with various important caveats of course, depending on your model.
[2]: Metal is supported on GPUs since Intel Ivy Bridge and Vulkan compatible AMD/Nvidia hardware. The Mac Pro 2010 is the exception since you can upgrade its GPU to a Metal compatible one.