I had a customer who was using the Campaign Manager plugin for Contact Form 7 and had started seeing an ugly error when they viewed their Plugins page in their WordPress admin portal:
Fatal error: Call to undefined function wpcf7_admin_url() in /home/xxx/public_html/wp-content/plugins/contact-form-7-campaignmonitor-addon/cf7-campaignmonitor.php on line 275
Contact Form 7 is a really powerful Contact Us-type form plugin for WordPress, which can manage multiple contact forms, and customize the form and the mail with a simple markup. It features Ajax-powered submitting, CAPTCHA codes, Akismet spam filtering and so on. I recommend it.
The Campaign Manager addon allows you to create newsletter opt-in forms with Contact Form 7, and automatically submit subscribers to predetermined lists in Campaign Monitor. It is pretty-well abandoned now, and hasn’t been updated for ages.
A smart bloke named Joel from a Authentic Design in the UK had worked out the
wpcf7_admin_url() function had been changed to admin_url(), but had also documented all the other pieces that had changed too!
Contact Form 7: https://wordpress.org/plugins/contact-form-7/
Campaign Manager Addon: https://wordpress.org/plugins/contact-form-7-campaignmonitor-addon/