Apple has a policy that apps (and their SDKs) must comply with IDFA, so if a user doesn't want to be tracked across the apps they use they can go to settings -> privacy -> advertising to turn off the tracking.
iOS doesn’t provide a system-wide unique ID however there is still more than enough data (WiFi network names, device name, device type, IP address, etc) that Shitbook can uniquely fingerprint a device and identify a user.