A brief explanation of cancel scopes for the uninitiated: A cancel scope can enclose just a part of a coroutine function, or an entire group of tasks. They can be nested within each other (by using them as context managers), and marked as shielded from cancellation, which means cancellation won't be propagated (i.e. raised in the coroutine function) from a cancelled outer scope until either the inner scope's shielding is disabled or the inner scope is exited or cancelled directly.

The fundamental problem in implementing these on top of asyncio is that native task cancellation can throw a wrench in these gears. Since a cancel scope works by catching a cancellation error and then (potentially) allowing the coroutine to proceed, it would have to know, when catching a cancellation error, if the cancellation error was targeted at a cancel scope or the task itself. A workaround for this, made possible in Python 3.9, is to (ab)use cancellation messages to include the ID of the target cancel scope. This only solves half of the problem, however. If the task is already pending a cancellation targeted at a cancel scope, the task itself cannot be cancelled anymore since calling cancel() again on the task is a no-op. This would be solved by updating the cancel message on the second call. The docs don't say anything about the behavior on the second call, so it's not strictly speaking a change in documented behavior.

Then, on the subject of level cancellation: level cancellation builds upon cancel scopes and changes cancellation behavior so that whenever a task yields while a cancelled cancel scope is in effect, it gets hit with a CancelledError every time, as opposed to just once in asyncio's "edge" style cancellation. Another very important difference is that with level cancellation, even a task that starts within a cancelled scope gets to run up until the first yield point. This gives it an opportunity to clean up any resources it was given ownership of (a connected socket in a socket server is a common, practical example of this).

This is what the asyncio documentation states about Task.cancel():

"This arranges for a CancelledError exception to be thrown into the wrapped coroutine on the next cycle of the event loop.

The coroutine then has a chance to clean up or even deny the request by suppressing the exception with a try … … except CancelledError … finally block. Therefore, unlike Future.cancel(), Task.cancel() does not guarantee that the Task will be cancelled, although suppressing cancellation completely is not common and is actively discouraged."

This is, however, only true for a task that has started running. A Task that gets cancelled before even entering the coroutine is silently dropped.

As asyncio does not allow for custom task instances without overriding the entire task factory, it leaves libraries like AnyIO some less desirable options for implementing level cancellation:

1. Implementing a parallel task system using low level synchronous callbacks (con: such tasks won't show up in asyncio.all_tasks() or work with third party debugging tools)
2. Adding callbacks to continuously cancel tasks that yield inside a cancelled scope (con: ugly; potentially extra overhead?)
3. Adding a wrapper for the task that acts as a "controller" (con: adds an extra visible stack frame, messes with the default task name)

Having low level machinery for injecting a custom Task instance to the event loop would probably solve this problem.