Expect to see many arches getting dropped as rust becomes a mandatory dep for an increasing number of common packages. Personally would have preferred for it to be optional so the cull was not necessary, however that is too much work for the devs.
He listed five architectures that are not supported by upstream Rust (alpha, hppa, ia64, m68k, and s390) and an additional five that are supported but do not have Gentoo Rust packages (mips, 32-bit ppc, sparc, s390x, and riscv).