Anything that makes cancellation decisions from the outside of a black box is unsound. You are asking the programmer of the thing being cancelled to program in

Anything that makes cancellation decisions from the outside of a black box is unsound. You are asking the programmer of the thing being cancelled to program in such a way, that the impact on the world outside abides by all invariants no matter where the cancellation happens. For threads, that an impossible ask. Thread cancellation is a big no-no. Just never do that. Sibling's mention of leaks and deadlocks are just examples of things that can go wrong. Task cancellation in async has the same issues. Less so, maybe much less so, because every space between two awaits acts as a no-cancellation-zone, but it's still a very suspect thing to do.