The canonical thing that works this way is Erlang (and its modern cousin, Elixir). See also "actor model" (e.g. Akka). It is approximately how Windows and Mac GUI used to work (back in the day; did not look at these APIs for ~20 years).

Python async is coroutines, a different kind of concurrency. In it, the event loop is hidden, and coroutines just yield control, implicitly or explicitly, to allow other coroutines proceed. In Python, a CPU-bound task can only run on a single thread, due to the Global Interpreter Lock preventing concurrent modification of data. Coroutines are still useful both for IO and as a general way to describe intertwined, mutually dependent computations. (The earliest, limited Python coroutines were generators.)

I'm not sure that's true, at least not in my problem space. Copying data leaves you the possibility of operating on stale data, which will result in the computation returning the wrong answer. To avoid that, you have to let the event handler know somehow when the data has changed. How are you going to do that?