When WP_DEBUG is enabled in wp-config.php, it will turn on the notices that deprecated functions were used. When WP_DEBUG is turned off or does not exist, then no reporting will be given.

This prevents the notices for everyday users who are uninterested in receiving these reports. As a consequence, only the users who actively enable WP_DEBUG will receive the information. Those users will not know to upgrade, unless they use a plugin which they download and activate.

The following code snippet enables WP_DEBUG and needs to be placed in wp-config.php.

define('WP_DEBUG', true);

Ref: wordpress.org