We\'re working on a project with my colleagues which involves using a lot of private and non official code. This is not intended for AppStore use.
The f
I don't know if commercial solutions are of interest to you but check out http://www.appsflyer.com
I'm not affiliated with them but we used their SDK at my previous employer. They have a device fingerprint technology which does work.
Note: If the user resets the IDFA then AppsFlyer will see this as a new device. However, and its been awhile so I can't remember, I think you can use their SDK, not use AdSupport.framework, and then they won't have the IDFA available to them. So I'm guessing that their device fingerprinting may work.
They also have competitors, search for device fingerprinting. Check out Yozio and branch.io, they both claim to do this. I've not used their product, just seen their websites.