Go doesn't use the Plan 9 calling convention, and there's not a single Plan 9 calling convention anyway. Go arguments are passed on the stack, and results are on the stack too. The Plan 9 C compiler used by the Go toolchain passes arguments on the stack, but the return value is in a register. On Plan 9, the first argument is usually (but not always) in a register. All these are different.

Go inherited and extended the Plan 9 toolchain, but the calling convention was changed. The main reason Go returns on the stack is to support multiple return values.