Last I checked, you could install Debian from a set of CDs which got regular point releases. Having something with decent power like a Core 2 Quad you could go back to around 2007 I guess (though I fear at this point they might have problems with their API). How do they solve the problem of running on Windows XP?
I suspect it's similar to how building a C++ project works. In Visual Studio you select the minimum version you want to support and you're done. This in general is a pain point of Linux - the toolchain is set up for building the system itself rather than producing binaries for general distribution and there is no easy way to set up a generic toolchain for a given basic version of libraries. This is great for letting you modify your system, but it's not ideal for software development.
Installing an old distro via docker or in a chroot works, but I wish I could have just a compiler plus libc rather than an entire Ubuntu. I think Redhat allows something like that, but it should be more widespread.