I think you need to split the unsoundness bug, which happens because thread::scoped uses unsafe code, from the "you can write mem::forget in safe code" bug, which is arguably not a bug but is at least an enormous footgun waiting to happen.

When the footgun goes off in unsafe code in the standard library, you get use-after-free and memory unsoundness. When the footgun goes off in safe code written by mere humans, you leak arbitrary resources.