This function has been deprecated. That means it has been replaced by a new function or is no longer supported, and may be removed from future versions. All code that uses this function should be converted to use its replacement if one exists. See also wp-includes/deprecated.php.
Use
any of these functions instead.
Note: This function is not defined in any WordPress core file since v3.1. It should be considered deprecated. It was added along with the other MU functions in 3.0, but was not being used so was removed.
Source File
show_post_thumbnail_warning() was located in wp-admin/includes/ms.php.