I logged into this very site, ran an update, and suddenly I see this error on my category pages:

category_link is deprecated since version 2.5.0! Use term_link instead

It looked pretty ugly and made me look like an idiot. Luckily I was able to find a workaround to carry me through until the next update, thanks to @whyisjake.

It looks like the WordPress team have deprecated a function prematurely, and they will probably undo the deprecation in an upcoming update.

If you have full write access to your WordPress files, you can banish the bug yourself by undoing the deprecation. It’s pretty straight forward, its just a matter of replacing two functions.

Beware, if you are not familiar with editing code – a stray letter here or there may cause your site to crash. You need to pay attention to the details when you are editing code. That said, it’s literally a matter of replacing two lines, so let’s get into it.

First, find the file we are going to edit, taxonomy.php which is inside your wp-includes folder.

Open that file in a text editor, and scroll down to line 4256, and you’ll find the first line we will replace:

$termlink = apply_filters_deprecated( 'tag_link', array( $termlink, $term->term_id ), '2.5.0', 'term_link' );

This is the line that’s causing the trouble. If you can’t easily find it, copy this into the search function to find it.

Once you have it, select it all, and paste the following line on it to replace it.

$termlink = apply_filters( 'tag_link', $termlink, $term->term_id );

The second replacement we’re going to make is exactly the same, the first line of code appears again on line 4268. Select it all once again, and replace it with the second line of code.

There, you’ve done it, you’ve rolled back the deprecation, and your error should have disappeared, if you’re facing the same issue I was.

Let me know how you get on.