Final answer:
In a secure password database, one should never store plaintext passwords due to security risks. Iterations, salt, and a password digest should be stored as they are part of secure password storage practices that protect against theft and misuse.
Step-by-step explanation:
When considering what should NOT be stored in a secure password database, the clear answer is a plaintext password. Storing passwords in plaintext is a severe security risk, as it would allow anyone with access to the database, whether legitimately or via a security breach, to see and potentially misuse the passwords. On the other hand, iterations refer to the number of times a hashing function is applied in the password hashing process, and storing this value enhances security by making the hash more resistant to attacks.
The salt is a random value added to the password before it is hashed to ensure that the same passwords will result in different hashes, which prevents the use of rainbow tables for attacks. Lastly, the password digest, which is the output of the hashing function (including the password and the salt), is what should be stored instead of the actual password, as it allows for password verification without exposing the actual password.