I can't find any documentation about this …
That’s because the documentation tends to focus on the behaviour that’s guaranteed, and there’s nothing guaranteed here. Imagine, for example, if you set up a download for a huge file and then the user never ran the app again. Should the system keep that download around for the next year? Clearly that’s less than ideal, but the process for cleaning up such downloads is very likely to change over time.
The current implementation puts downloads into the Caches directory, and thus they get cleaned up via the standard mechanism used to clean up caches when the device runs low on disk space.
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Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware
let myEmail = "eskimo" + "1" + "@apple.com"
Yes, indeed – I do appreciate the difficulty you have, because if you tell us enough about the implementation for us to understand what you mean by the feature, we tend to go and assume that this is part of the contract, with all sorts of recriminations when it turns out that it wasn't. So you're damned if you tell us anything and damned if you don't!
But thank you for a clear explanation of the general idea. "Same as other caches, for now at least" is a reasonable policy and it is possible for us to think about our app with that in mind (though not assuming it to be a contractual guarantee).