s390 is the 31-bit-only variant, that has been discontinued for some time. Modern variants are 64-bit based, and still supported.
All that being said, it's quite worthwhile to include these "dead" architectures in LLVM and Rust, if only for educational reasons. That need not imply the high level of support one would expect for, e.g. RISC-V.
Two architectures currently being added to LLVM are m68k and csky. I don't think either are that new (I thought csky was, but it was explained to me by Linux kernel architecture folks that it has old roots from Motorola, with folks from alibaba using that for 32b but moving to riscv for 64b).
Could you expand on that? Are you saying that s390x can run binaries compiled for s390 and that today binaries are being compiled to s390 for the purpose of being run on s390x?
Yes to both (at least for user mode code, or "problem mode" in IBM parlance. Kernel and hypervisor code is 64-bit only on newer chips). There's something like a 30% average memory savings for 32-bit code, so if your program its in 2GB, it's a win on these massive machines that'll be running 1000s of VMs at close to100% load. Nice for your caches too.
All that being said, it's quite worthwhile to include these "dead" architectures in LLVM and Rust, if only for educational reasons. That need not imply the high level of support one would expect for, e.g. RISC-V.