Low-level firmware, embedded applications, OS kernels, interrupt service routines, applications that can't afford the latency of being periodically interrupted, platforms where you don't have a scheduler or most OS services, libraries intended to be loaded into other applications written in other languages (e.g. where you don't control the main program entry point), writing language runtimes themselves, etc.

Early Rust, pre-1.0, did have a runtime and a green-threads mechanism; they ripped it out because they recognized that they couldn't go everywhere they wanted to go if they kept it. And if they hadn't done that, I believe Rust would have been far less successful than it has been.