iOS: Using Keychain to Get a Persistent, Cross-Install Device Identifier

I don't see any reason why Apple wouldn't allow you to rely on some id that only makes sense for your use case and can't be used in any malicious intent. Also see my reply to therealidiot.

I would say that this approach is 100% allowed by Apple, as it doesn't allow device tracking between different apps (specially if you skip the IFA step). That was the main concern of Apple, that lead to disallow developers from accessing UUDID, mainly because it allowed advertising services to cross usage data between different apps.

Using Keychain to store your UID just ensures you don't lose your UID, not a system-wide UID.

Using the KeyChain approach is perfectly valid since you are just making sure your own unique_id, which has value only to you, is never lost. You are never storing an id which might be used with malicious purposes, which was the intention of Apple in the first place.