Deleting an app’s data from the iOS Keychain, or resetting the Keychain entirely I installed an app that stores a persistent identifier in the iOS keychain, even after uninstalling and reinstalling it. This really bugs me. Is there a way to delete said app's data from the Keychain, or worst-case resetting the entire iOS Keychain, without resetting the device completely?

Note: I'm not talking about the iCloud Keychain, but the on-device keychain that apps can store small bits of data in.
Have you tried this link? I'm pretty sure this is the answer you're looking for, this link tells you to; 


  1) Open the Mac Keychain. Click Finder > Go > Utilities > Keychain
  Access.
  
  2) In Keychain Access, select the Passwords category on the left to
  review the list of sites and services for which you've saved a
  password.
  
  3) Right-click the service you want to edit/remove, then left-click
  Delete.


https://answers.uillinois.edu/68546

If you're looking to get rid of sandbox, 


  1) Quit sandbox as well as its related process(es) if they are still
  running.
  
  2) Launch Finder on your Mac, and click Applications in the Finder
  sidebar.
  
  3) Search for sandbox in the /Applications folder, then drag its icon
  to the Trash icon located at the end of the Dock, and drop it there.
  Also, you can right-click/control click sandbox icon and then choose
  to Move to Trash option from the sub-menu.
  
  4) If prompted, enter the administrator password and hit OK to allow
  the change.
  
  5) Choose Finder > Empty Trash from the Menu bar. Or choose
  right-click Trash icon, choose Empty Trash, and Empty Trash again in
  the pop-up dialogue.


https://macuninstallguides.com/remove-sandbox-on-mac.html


  (Note, you don't have to install any apps to remove it)


If you want to reset the entire keychain, I recommend that you take a look at the answer on this page.

How do I delete or fix my Keychain after changing my password?