I can confirm that old recovery keys are added to the iCloud Keychain, even if you explicitly opted out of iCloud recovery before. This is exactly what happened to me when I upgraded my systems to macOS 26 yesterday.

iCloud Keychain is NOT the same security as a hardcopy written down recovery key, which is what I used before. This is absolutely a forced change in security policy that was not communicated or opted into by the user.

Was iCloud Keychain enabled before you upgraded? Or was it forced on?