The developer - or customers - may wish to restrict or eliminate use of a function, library, or third party component for any number of reasons, including real or suspected vulnerabilities.

difficulty to use securely.

export controls or license requirements.

obsolete or poorly-maintained code.

internal code being scheduled for deprecation.

etc. To reduce risk of vulnerabilities, the developer might maintain a list of "banned" functions that programmers must avoid using because the functions are difficult or impossible to use securely. This issue can also make the product more costly and difficult to maintain.