Deleting an app’s data from the iOS Keychain, or resetting the Keychain entirely
Solution 1:
No. Application sandboxing means these entries are stored in a way other third party apps cannot reach into iOS sandbox library files to observe or clean. I suppose you could jailbreak the device if you had to purge things.
I totally get the annoyance - it’s your data and your device, so it’s frustrating to not be able to control it, especially since cookies and local store is a privacy issue persisting data when an app is removed.
The only practical solution short of a jailbreak into third party app sandboxes is to use apps that store items in an iCloud synced keychain so a Mac can clean and observe.