For example, on unstable the support for anything below SSE4.2 was recently quietly disabled by updating numpy (see this issue).
I tried googling but couldn’t find any official information. What are the hardware requirements for NixOS at the moment? What will they be in ten years?
In theory, packages built for the default x86_64 target shouldn’t rely on any additional CPU features. Packages that need such things, in theory, ought to be testing things like stdenv.hostPlatform.sse4_2Support and setting compilation flags appropriately.
In practice, things that only break computers from 2008 will occasionally slip through the cracks, especially on unstable; and I’d bet a box of donuts that at least one of our hundreds of maintainers has at some point intentionally written a derivation that assumes x86_64-v2 or something, in order for the cached build of their package to be a few percent faster, and hoped nobody would notice. 
I understand, thank you for the quick response.
Then I’ll try pushing for numpy to support all x86-64 CPUs again.