Non existing Callbacks One of the nice things is, that neither do_action(), nor apply_filters() do trigger an error if a callback isn’t present. This means that its the safest way to insert plugin data into templates: If a plugin is turned off and do_action()/apply_filters() don’t find the callback in the global $wp_filters array, nothing happens. … Read more
In addition to the answer by @Welcher: There are some good “graveyard” examples in the core, where “functions come to die“. You could use them as guidelines, e.g. regarding the documentation. Here’s one such example for the permalink_link() under the wp-includes/deprecated.php /** * Print the permalink of the current post in the loop. * * … Read more
To the best of my knowledge, and having heard Nacin say it explicitly, no deprecated function has ever been removed from WordPress to date. No guarantees for the future, but it’s never happened yet. Still, avoid deprecated functions.