A Reddit user shared a long article explaining how iOS 14 might retain your user data even after removing certain data collection apps. He gave an example with 9gag but the behavior is probably not limited to it.
It found user data stored on the local keychain and used again after you reinstall the app. The discovery was made after removing and then installing the 9gag app, which did not prompt any credentials requests and automatically logs you in.
The stored data is here to stay even if you don’t sync with iCloud Keychain and the only way to get rid of it is to perform a factory reset. However, the data can be transferred back to your device from iCloud Keychain when you restore your backup. The user likened the behavior to a super cookie which you cannot get rid of.
This does not mean that app data on the device is used to track or target you, but it may be a privacy responsibility and it would be nice for Apple to provide the ability to completely erase your user data if it wishes to maintain its good privacy record.
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