This is a perfect solution fallacy. Yes, "whether this function will suspend (and shift execution onto a different thread)" does not tell you absolutely everything about that function. But it tells you things that are worth knowing, that you want to be visible (not super intrusive, but visible) in your IDE. And in a language with a decent type system the cost is pretty small; functions that are agnostic about whether they will run as async can and should simply be polymorphic in async-ness.