App that retains data between installations. iCloud Keychain has never been enabled I noticed that Pinterest app on my iPad (iPad 4, iOS 10.3.3) keeps log in credentials after delete/reinstall. As a iOS user and not a programmer, I managed to find out from some posts I could understand, that iOS apps can keep data between instalations on Keychain, iCloud or Dropbox. 

Pinterest never asked for access on iCloud, Dropbox or similar cloud storage. I suppose it stocks login data in Keychain and do not delete it when uninstalling. 

Please help me to understand: is it a local keychain that iOS apps can use, different from iCloud Keychain? Is it so that only the iCloud Keychain could be user enabled / disabled, the local keychain beeing an always on iOS feature that every app developer can use whithout requesting user permission? 

I ask this because I never enabled iCloud Keychain on any of my iOS devices. Or does Pinterest app use other iOS feature that I don't read about to retain log in credentials?
iOS apps can keep important data (like login credentials) in the local Keychain. It is not the same keychain as the iCloud Keychain. iCloud Keychain is located in iCloud. Local keychain is located on your device, will never leave your device, only when you backup your iOS device and only when those backups are encrypted.

When you delete an app from your iOS device, there isn't really a uninstall procedure that also deletes keychain data. 

What you could try is to sign off in Pinterest before you delete the app. If the Pinterest app is developed properly the sign off function should delete the login credentials. (not sure about it)