The data is stored locally on the smartphone and is used when reinstalling applications. Only a reset will help to completely remove them.
One of the Reddit users with the nickname blackmolecule shared a story that iOS 14 keeps user data of applications even after they have been deleted from the device. He gave an example with a mobile client of the popular humorous platform 9gag, but the situation, apparently, is not limited to individual applications.
The editor discovered that the smartphone contains data that can be reused after reinstalling the application. He made this conclusion after uninstalling the 9gag application. After installation, it automatically authorized it without asking for credentials. At first, he thought the app was storing data in iCloud or keychain. There was nothing in the cloud storage, and there was no mention of 9gag in the keychain.
This way, apps can store data locally, and iOS doesn’t delete it even if apps don’t sync to iCloud. The only way to fix this problem is to do a full factory reset and check your iCloud backups and keychain for any traces at the same time.
Redditor compares this behavior to cookies, which cannot be eliminated without a complete reset of the device. For some users, this can be a serious privacy issue. Of course, this does not mean that the app data on the device is used to track users, but it would be appropriate for Apple to make it possible to completely erase it when apps are uninstalled.