Looking at the source code, it seems that Alamofire chose not to expose the underlying automaticallyCancelling for serializingDownload(using:automaticallyCancelling:).
I know that I can extend DownloadRequest and make a serializingDownloadedFileURL(automaticallyCancelling:), but I'm wondering if there is a reason related to Swift Concurrency that I'm missing?
It's really to follow the principle of least surprise, as there are several areas where concurrency constructs can implicitly cancel where the user may not expect their request to be cancelled at the same time. So it's better to opt into the behavior rather than have to opt out.