Creating directory within Application Support, and preventing iCloud backup
File System Basics tells us:


  The contents of the Library directory (with the exception of the Caches subdirectory) are backed up by iTunes and iCloud.


And


  Put data cache files in the Library/Caches/ directory. Cache data can be used for any data that needs to persist longer than temporary data, but not as long as a support file. Generally speaking, the application does not require cache data to operate properly, but it can use cache data to improve performance. Examples of cache data include (but are not limited to) database cache files and transient, downloadable content. Note that the system may delete the Caches/ directory to free up disk space, so your app must be able to re-create or download these files as needed.


If this content is readily re-downloadable, Caches might actually be a reasonable idea. It makes your app a “good citizen”. If the user is running low on storage, it might be better to allow re-downloadable content to be removed rather than tempting the user, looking at their storage taken up by various apps, to contemplate removing your app because it’s taking up a lot of unreclaimable space.

If you want to ensure the content will never be deleted when the device runs low on space, then go ahead and put it in the Application Support directory, but manually exclude it from backups, like you have done.

Feel free to refer the iOS Storage Best Practices video (and that page has useful links).



You said:


  I think there's no guarantees about the lifetime of files in the Cache dir


Correct. In practice, when (a) the device runs low on space; and (b) your app is not running, the OS can purge this directory. It starts with apps that have been use less recently.


  Also, I want to make sure that those content files aren't backed up to iCloud or anywhere else; and that they're not accessible by the user or other apps.


By the way, whether it’s backed-up or not and whether it’s accessible by the user are two different questions. E.g., files in the Application Support directory are backed-up (other than, obviously, content of the Caches folder or those files explicitly designated to not be backed-up). But the contents are not visible to the user. Only user documents stored in Documents folder or iCloud Drive are visible to the end-user.

So, if your content is not readily re-downloadable, but you don’t want to expose them to the individual files, Application Support (without specifying NSURLIsExcludedFromBackupKey) might be prudent. It’s your call.