async/await was never a compatibility break at the CLR level. Most of async/await is "just" the TPL (Task Parallel Library), a subset of the .NET BCL (Base Class Library) and entirely handled as libraries, which any language can use/interact with even without async/await language support.
The async/await language support in C# is basically a Monad transformer, building a generator-like asynchronous state machine. (The output of an async/await function in C# is rather similar in concept/implementation to the output of a generator function using yield syntax.) This is also why F# has never had a tough time with async/await because it has even more generic Monad transformer support ("computation expressions").
Any "alt-lang" could make use of the TPL whether or not they could transform their own similar syntax, and there's nothing stopping them from building a similar syntax. As far as I saw that wasn't a killer of languages.
It wasn't a break at the CLR level, but it was a break at the BCL level. To consume the new async methods your language had to come up with a mechanism that was broadly similar to how C# worked or miss out on the vast majority of libraries and the BCL itself.
If a language didn't do anything at all like C# it dealt with a lot of task.ContinueWith(value => {}) callbacks (Action<T>), no different to JS promise.then(value => {}) chaining (truly, no different at all because Task/Promise are basically twins). At worst case you had callback "waterfalls" to deal with, but that was hardly "breaking" and shouldn't have "killed" languages. async/await is much nicer than callbacks/callback waterfalls, but that doesn't mean you can't write callbacks/callback waterfalls (a lot of JS projects certainly did; many still do even despite most browsers supporting async/await today).
The language would also need to have some support for anonymous delegates and closures, or its callback waterfalls would have to be broken up into multiple methods that would have to manage the call state themselves.
CLS really expected most languages to look a lot of like early C#, that is, Java with some Delphi/VB features grafted on.
The async/await language support in C# is basically a Monad transformer, building a generator-like asynchronous state machine. (The output of an async/await function in C# is rather similar in concept/implementation to the output of a generator function using yield syntax.) This is also why F# has never had a tough time with async/await because it has even more generic Monad transformer support ("computation expressions").
Any "alt-lang" could make use of the TPL whether or not they could transform their own similar syntax, and there's nothing stopping them from building a similar syntax. As far as I saw that wasn't a killer of languages.