AMD's definition of "support" I think is different than what people expect, and pretty misleading - ROCm itself will run on almost anything, back as far as the RX 400/500 series:
There are out-of-bounds writes in the BLAS libraries for gfx803 GPUs (such as the RX 570). That hardware might work fine for your use case, but there's a lot of failures in the test suites.
I agree that the official support list is very conservative, but I wouldn't recommend pre-Vega GPUs for use with ROCm. Stick to gfx900 and newer, if you can.
The last time I checked, I was stuck with a pretty old kernel if I wanted to have the last version of ROCm available for my rx470. It's compatible at some point in time, but not kept compatible with recent kernels.
https://en.wikipedia.org/wiki/ROCm#:~:text=GCN%205%20%2D%20V...
Stable Diffusion ran fine for me on RX 570 and RX 6600XT with nothing but distro packages.