But whether a function is not async-safe is pretty black and white (i.e. there's a lot of clearly unsafe code). Even a definition as simple as "performs blocking I/O" would be extremely helpful.

This is the value people get out of async only environments like node. Yes, you still have to worry about costly for loops but I don't have to worry about which database driver I'm using because they are all async. In a mixed paradigm language like rust I would really appreciate the compiler telling me I grabbed the wrong database driver.