I'm not sure about this and I don't think it's equivalent to what you are proposing.
With async let you create something known in other environments as a promise. Nothing is executed until you actually call the promise with await where you are back to square one, it's just a plain await call in the end.
Your proposal is a fire-and-forget kind of an async call where you are not interested in the result and therefore you want to continue execution of the current context without awaiting. It's a totally different thing.