Javascript's async/await probably started as a sugar for callbacks (since JS is single-threaded). Many others definitely have that as sugar for whatever threading implementation they have. In C# it's sugar on top of the whole mechanism of structured concurrency.
But I'm mostly talking out of my ass here, since I don't know much about this topic, so everything above is hardly a step above speculation.
Javascript's async/await probably started as a sugar for callbacks (since JS is single-threaded). Many others definitely have that as sugar for whatever threading implementation they have. In C# it's sugar on top of the whole mechanism of structured concurrency.
But I'm mostly talking out of my ass here, since I don't know much about this topic, so everything above is hardly a step above speculation.