that link is really just to a plugin.
That appears to be correct.
That plugin would seem to be intended to prevent the nag from showing, should you decide to continue to use the plugin that places the banner at the top of the admin panel. Looks like it might try to do that by removing the function that places the banner in the admin area. That function can be found in the plugin files themselves.
In the examples I looked at, the function is clearly defined and easy to locate. It wasn’t hidden or obfuscated in any way.
/* -------------------- Update Notifications Notice -------------------- */
if ( !function_exists( 'wdp_un_check' ) ) {
add_action( 'admin_notices', 'wdp_un_check', 5 );
add_action( 'network_admin_notices', 'wdp_un_check', 5 );
function wdp_un_check() {
if ( !class_exists( 'WPMUDEV_Update_Notifications' ) && current_user_can( 'install_plugins' ) )
echo '<div class="error fade"><p>' . __('Please install the latest version of <a href="https://premium.wpmudev.org/project/update-notifications/" title="Download Now ?">our free Update Notifications plugin</a> which helps you stay up-to-date with the most stable, secure versions of WPMU DEV themes and plugins. <a href="https://premium.wpmudev.org/wpmu-dev/update-notifications-plugin-information/">More information ?</a>', 'wpmudev') . '</a></p></div>';
}
You can also remove the nag simply by commenting out or removing the code in the plugin file. The plugin is just meant to be a convenient method of doing the same thing without the need to edit a file.
I installed and activated the plugin I took this code from, and the update nag banner showed as described. I then deactivate the plugin and the banner went away.
I ran a differential on the database: copy one, a backup I took immediately prior to installing the plugin, copy two, a backup I took immediately after deactivating the same plugin. I didn’t find anything unusual in the database from the plugin, and experienced no residual effects from the plugin in the dashboard after deactivating it.
That being said and done, and without the benefit of having you name the plugin that caused the issue or elaborating on other conditions that could have caused the banner to persist (ie.. did you in fact at any time install the suggested update plugin? You don’t say one way or the other) the update plugin itself, could be an entirely different story.
I’m not sure how much further one could go in trying to duplicate your issue with the information provided. It’s just not a WordPress issue. It’s a plugin developer business model issue. It’s also a topic that’s present in detail, and with frequency, in their own support discussions.
Given all of that information to work with, is there anything that anyone here can do to help any further with the issue?