Hello telyya and welcome to the Apple Support Community.
The old iPad no longer appearing in your Apple Account device list does not necessarily prevent its former passcode from being requested. A previous device passcode can be used to authorize access to end-to-end encrypted iCloud data.
The reply above describes the general reset principle, but your detail is important : if “Forgot iPad Passcode?” currently gives you only “Try Again Later,” then the reset/delete option is not presently being offered on your iPhone.
→ We would proceed without repeatedly guessing the old passcode :
• Update the iPhone, restart it, then return to “Some iCloud Data Isn’t Syncing” and select “Resume Data Sync.”
• Enter the current iPhone passcode first if requested.
• If another previous trusted device is offered, you can use the passcode for that device.
• If “Forgot all passcodes?” or “Delete iCloud Data” becomes available, read the warning carefully before confirming.
Resetting end-to-end encrypted data can permanently remove encrypted information that cannot be recovered from another trusted device.
If the only choice remains “Try Again Later,” we would contact Apple Support rather than sign out of iCloud, erase the iPhone, or repeatedly enter guessed passcodes.
Apple cannot decrypt or recover inaccessible end-to-end encrypted data, but Support can help determine why the reset option is not being presented.
If you contact Apple Support, please come back and let us know whether the reset option could be restored.
🙏
Have a good day.
And one more thing…
Never forget to think different.
Important :
Always make sure that your Apple devices are running the latest version available of their operating system and that your apps are updated to the latest version available in order to benefit from fixes and compatibility improvements.
Useful links :
Set up iCloud Keychain – Apple Support
iCloud data security overview – Apple Support