I actually wanted to be less invasive than asking people to sign up. If you want to sell your users data then getting them to sign up is the way to go, but all I want to do is provide customization for a user that's frictionless and more private than an email based account. All I would know is that anonymous user identified by what to me is a random guid likes x,y, & z then I would deliver a customized feed. signups are so annoying for users. Zillow seems to give me a customized experience and I've never signed up with them, they do ask for push notifications though so I assume they are leveraging that as the user's moniker.
I wouldn't expect to be able to get the UDID, but a trivial solution would be to hash UDID and the app's GUID (the app store must have some ID for every app that's unique) then sending this to any server, harvesting it for any reason would be completely useless except for that app, it would mean that that app alone would be able to track you, which is safe for the users and great for the app developers because it allows for customization while providing the user privacy.
Phone number I totally get why Apple would not want that, as a user I wouldn't want that either.