Apple allows apps to share information between them without asking permission
1. Logged in Linkedin? the SlideShare app already knows who you are. 2. Logged in any Google app? all other Google apps know who you are. 3. Facebook + Instagram, Google + Waze, etc...
That is ultra-annoying, besides login, apps can share information between them without you knowing it, being able to disable it, or sometimes even know which apps can share between them (do you remember that LinkedIn bought Slideshare? or Facebook bought Instagram) ?
Some apps don't even require login (such as Waze), but might (who knows) link some cookie to your other Google apps...
Just like in the open web where you can open different profiles in your browser (or even use incognito mode) to separate information passing between sites, iOS does not give you this option, and does not notify you when apps talk between them.
If Apple is serious about privacy, it seems basic to me to notify a user when this is about to happen and have an option to disallow it exactly how it does for all the other Privacy policies it has (location, contacts, etc...).
[1] Sharing Keychain Items https://developer.apple.com/library/content/documentation/Security/Conceptual/keychainServConcepts/02concepts/concepts.html
"In iOS, and in macOS,..., you share keychain items using Access Groups. This kind of sharing does not require interaction with, or permission from the user, but limits sharing to apps that are delivered by a single development team."