Inspired by this question on WPSE, I decided to create a plugin that scanned defined directories for deprecated functions.

Deprecation Checker collates a list of deprecated functions directly from the installed WordPress core files. By default, the themes and plugins directories are scanned automatically. I’ve included a couple hooks for developers to extend the plugin for their own uses by adding custom paths to scan (recursively) and custom deprecated functions to look for.

Once you install the plugin, simply browse to the Tools administration menu and start scanning your files. It might amaze you how many of your installed plugins and themes are out-of=date!

Download the Deprecation Checker Plugin