Technically green threads were removed when rust was moved “down the stack” back before 1.0: green threads would mean requiring a runtime (scheduler & al) which is problematic when targeting foundational libraries and embedded programming, it makes integrating with other systems a lot harder.

Async being opt-in means the people who don’t need it can avoid it, though it’s not always great when e.g. the premier HTTP client of the ecosystem provides a “blocking” interface which iirc just starts a runtime and delegates to the async interface.