I run Arch Linux on my M1, is that not arm?

No, you run an Arch derivative.

> Arch Linux is an independently developed, x86-64 general-purpose GNU/Linux distribution that strives to provide the latest stable versions of most software by following a rolling release model.

- https://wiki.archlinux.org/title/Arch_Linux

> This page complements the Installation guide with instructions specific to Apple Macs. The Arch installation image supports Apple Macs with Intel processors, but neither PowerPC nor Apple Silicon processors.

(emphasis mine)

- https://wiki.archlinux.org/title/Mac

(FWIW, I understand that there is benefit to good coverage of a narrower scope, but I do wish Arch would fold https://archlinuxarm.org/ into the main project and be officially multi-arch, but that is not the world we live in.)

Arch package manager here, there is ongoing work behind the scenes to support multiple architectures (aarch64, riscv, etc), but as our volunteers (myself included) are doing this in our free time, progress is up in the air.

That's great to hear:) Given the long-term existence of eg. https://archlinux32.org/ and https://archlinuxarm.org/ I had always assumed that this was purely a question of policy and that Arch had no interest in supporting anything else. I found https://rfc.archlinux.page/0032-arch-linux-ports/ ; is there anything else I could read to catch up on the state of things?

Core arch linux doesn't support it, it's an offshoot.