It is no longer necessary to use this to achieve runtime-less operation;
just use #[start] and/or #[no_std] and avoid the runtime.
But "#[no_std] no longer works"
I think zero.rs should keep example implementation of runtime-less Rust and update it as things change in Rust. In "good times" it should be some short thing with just #[no_std] and #[start], in bad times it should also contain various workarounds to make it working.