Why mips and not RiscV? I would assume it’s easier to emulate in software and has good support in linux

EDIT: found it

Some architectures had arbitrarily-shifted operands all the time (ARM), some have shitty addressing modes necessitating that they would be slow (RISCV), some would need more than 4KB to even decode instructions (x86), and some were just too complex to emulate in so little space (PPC).

Could it be the pc relative addressing often used on risc-v would be slow to run on 4004?

Funny you mentioned arch, as steam deck os is based on arch, so it is using arch btw