"iForgot is asking for the passcodes to old devices I no longer own: iForgot is asking for the passcodes to old devices I no longer own to reset my password instead of prompting me on the trusted device from which I am submitting the request. The old devices are NOT listed as trusted and NOT listed on the Find My device screen. How do I remove these old devices from the list so that iCloud stops prompting for passcodes from non-trusted, old devices?"
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Stopping Getting Password Prompts:
A. Change your Apple Account Credentials:
Someone is trying to log into your account on these devices (either because they were stollen, or you did not rid of these properly). So, start with changing your Apple Account info, starting with a new password. A step up from that would be to assign a new email address for your Apple Account.
B. Remove these from your Account:
The device(s) is still associated to your Apple Account. Go here to see if you can remove it: View and Remove your Devices that are Associated with your Apple Account for Purchases - Apple Support
C. Properly Ridding of These:
When you rid of a device, then you need to perform as instructed here, including the backup, and iCloud logout: