AMD ISAs are changing for almost every generation so LLVM[1] continues to keep the architecture name "amdgcn" and handle the variation based on the model flag (e.g., -mcpu=gfx1030 for RDNA2, -mcpu=gfx1100 for RDNA3).

> I though that these GPU ISAs were "proprietary"

PTX spec[2] is publicly available but the actual hardware assembly (SASS) is not. Although i believe Nsight allows you to view it.

1. https://llvm.org/docs/AMDGPUUsage.html#processors

2. https://docs.nvidia.com/cuda/parallel-thread-execution

If LLVM can target AMD GPUs what exactly prevents AMD and ROCm from supporting all the damn GPUs?

At this point I'm convinced that the real problem with AMD GPUs isn't necessarily the compilers (although they do produce mediocre code) or even the hardware itself, but some crappy C++ driver code that can't handle running graphics and compute at the same time. The datacenter GPUs never had to run graphics in the first place, so they are safe.