An iPhone 6 Plus' Touch ID stopped working and the user couldn't remember the passcode and, after many failed attempts, was forced to choose between setting up as a new iPhone or restoring from backup.
The iPhone 6 Plus was frequently backed up by iCloud and never backed up by iTunes.
Directly (from the initial setup) or indirectly (once setup as a new iPhone), attempting to restore from iCloud prompted for and accepted Apple ID authentication (username, password, and phone number-based TFA) then prompted as follows and rejected all given:
Enter Old Passcode
Passwords, home and other data are protected by the passcode previously used to unlock this iPhone.
Your passcode is encrypted and cannot be read by Apple.
I wasn't able to find anything online regarding this scenario, hence this post.