ARM seems to have pretty thoroughly won the category of “embedded targets that are capable enough to run Linux”, though. Other than ARM and (presumably in the future) RISC-V, what other architectures are still relevant for new embedded designs that would run Linux?

Obviously there is still significantly more diversity at the low end of the embedded world, but you wouldn’t run Linux on e.g. an MSP430, even if you somehow had a compiler that would support that.

This is why the idea for Rust in the kernel, for now, only targets drivers. Drivers are target specific, so you can use it only for drivers on architectures that are LLVM capable.