WordPress core code contains things marked as deprecated by… Wordpress?

Question

I have some problems matching this message in my WordPress network update page:

You have the latest version of WordPress. Future security updates will be applied automatically. If you need to re-install version 4.9.4, you can do so here:

With the one I just noticed after temporarily enabling debug mode:

Notice: wpdb::escape is deprecated since version 3.6.0! Use wpdb::prepare() or esc_sql() instead. in /home/user/public_html/blog.network/wp-includes/functions.php on line 3846

Notice: get_current_site_name is deprecated since version 3.9.0! Use get_current_site() instead. in /home/user/public_html/blog.network/wp-includes/functions.php on line 3846

What am I missing? How can core WordPress code still use code that its very own developers marked as deprecated? How can I update functions.php file in wp-includes to not use deprecated code and methods?

Answer 1

Line 3846 in wp-includes/functions.php is in a function named _deprecated_function(), which warns about the use of deprecated functions. I did a quick search of my copy of 4.9.4 core and didn't find any instances of wpdb->escape() or get_current_site_name(), the two functions you're actually being warned about.

Most likely, the deprecated functions are in a plugin or a theme. If you disable all plugins and switch to a default theme (such as Twenty Seventeen), the Notices should go away.