I’ve been just migrating my setup for every new iPhone since maybe the 5. I have a sick feeling that in spite of what we’re told, if if looked at the actual file system under the good I’d find like 1500 plist files and similar cruft? 
If you look at iCloud in terminal on a Mac you’ll see remnants from every app you’ve ever installed that used iCloud. And I’ve got music and movies in my Apple collection that I didn’t purchase and was never charged. When I asked about it Apple support said they couldn’t remove them.
I’d be surprised if there wasn’t garbage left over from previous activity. So I don’t worry about it. Apple doesn’t or they would do something about it.
Curious. How do you do this?
Open Terminal, at the prompt type:
cd Library/Mobile\ Documents/
and press Enter
Then type:
ls -al
and press Enter:
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Unlike macOS, nothing is left behind after you uninstall an iOS app. Each app is sandboxed into its own section of the disk and memory, and can’t escape it due to the system design. So it has to store and use all of its files/preferences inside that sandbox. When an app is deleted, that entire sandbox is removed.
The only exception I’m aware of was, for awhile, for iOS to leave some inaccessible keychain items behind, but I believe that’s been fixed for years, and they weren’t transferred to new phones anyway.
It’s not just what we’re told; it’s what thousands of iOS devs have tried and failed to break for years now. If/when a vulnerability is found that lets an app affect the rest of the file system, it tends to make the news and is patched.
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This is my understanding as well. Essentially, each app thinks it has the whole machine to itself.
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That’s very good to hear, I’ve read the same, just wanted some additional confirmation, thanks!
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But….could the app even clean that up? The app doesn’t get a dialog at delete time, and there’s no guarantee that data isn’t being used elsewhere.
My understanding is that the complete app and non-iCloud data get wiped clean, and anything put in iCloud ceases to be part of the app and is thus not auto-purged.
I don’t know. I’ve always dug into new operating systems to understand their structure and purpose of the various bits. (I must have trashed and reinstalled Windows 95 at least once a day the first week it was available).
So when I first saw a behind the scenes view of iCloud it bothered me that I couldn’t clean up the leftovers. But IOS has always been locked down and now it’s happening to the Mac, so I’ve quit worrying about it.