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.
Not joking at all, I still use it daily, works fine as a machine for browsing the internet and watching youtube/netflix, with ocassional dev in terminal. Replaced the HDD with an SSD years ago and it's still very brisk. It runs El Capitan 10.11.6, which I don't have any issues with, but I am mildly concerned about the lack of security updates for it.
Sometimes I use a quad-core laptop with 2 GB of memory and no swap (because it doesn't have enough disk space for a meaningful amount of swap) and five tabs is practically a hard limit. Even with adblocking, news sites and sites like imgur or reddit -- or really anything using heavy and fancy scripting or animations -- demand to be the sole tab on the machine.
Just speaking of performance, I can easily see systems where there's about a five-year gap before a low-end system catches up in performance to a higher-end one. e.g. a 2010 iMac performs about the same as a 2015 12" MacBook, though the latter will retain support to a much later date.
