Linux

You can still compile a surprising number of modern programs and libraries without unicode support (that is, they provide an explicit compile flag to switch it off)—it's just that no general-purpose distro does it by default. I'm not sure you can set up an entire unicodeless system using current software versions, but I wouldn't bet against it, either. And glibc isn't the only game in town—musl is viable and modern (it's the default libc in Alpine Linux and an option for some other distros), and designed for resource-constrained environments. Those two things between them might bring down the size by considerable.