I'm a Linux noob, and I was under the impression that Linux was written in C, and this could be compiled for any architecture, given a compiler with support for it. This really shakes my faith in Linux, if architectures, and even individual CPUs, need to be special-cased in the codebase.

Just kidding. I think I know what's going on here, and it's actually worse than that. Big company (IBM) pushing for tighter integration with their new chips (POWER10), which results in the kernel maintainers having to drop support for their old chip. Why they let a big company bully them like that is beyond me, but I guess something something IBM RedHat Kernel maintainer salaries something. :)