iOS apps persist keychain data even after the app is deleted.
For example, this allows Facebook to track you across multiple installations even if you intentionally uninstall the app.
There is no way to delete this data without jailbreak.
IMHO this is a huge privacy issue that needs more awareness.
source: https://forums.developer.apple.com/thread/36442
example project: https://github.com/kishikawakatsumi/KeychainAccess/tree/master/Examples/Example-iOS