Deprecated Functions in Wordperss CMS

WordPress is increasing its functionality and its features so after a new version of WordPress is being developed, certain functions may become deprecated. Deprecated function means the function is not removed from WordPress, while it should not be used in your themes and plugins. In this a situation a new function has been created in wordpress to replace the deprecated function or existing function. There are many reasons for this deprecated function in WordPress, Function needs a complete rewrite to better handle and give better performances to the feature it adds to WordPress so this is the most common reason for deprecated functions.
The WordPress CMS has a file to store all functions that have been deprecated over the age of wordpress. The best and organize way of the wordpress CMS so it is known as having superior backwards compatibility. It means that during the new version of Wordperss CMS released have a strong focus it put on backwards compatibility to verify new features and functions will not break existing sites running WordPress, below a inline documentation for the get_current_theme() deprecated function:
/**
* Retrieve current theme name.
*
* @since 1.5.0
* @deprecated 3.5.0
* @deprecated Use (string) wp_get_theme()
* @see wp_get_theme()
*
* @return string
*/
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Here you noticed that the few additional comment lines for deprecated functions are there. @deprecated is the first line stating and it tells that in what version of WordPress the function was deprecated, here the v.3.5. And the second word @see it telling you what function should be used instead after that the function name mentined above case is wp_get_theme().
So the deprecated.php file is a very important file to check when a new version of WordPress is released, beacasue it has all the function which has chaned. If a common function has been deprecated in the wordpress letest version, you immediately stop using it and even consider updating your old code to use the replacement for this function.
The deprecated functions gernally not removed from WordPress core files, but there is no guarantee that a deprecated function won’t be removed in a future release new version of WordPress.