PHP 4 style constructors are being deprecated in WordPress 4.3 to ease the transition to support PHP 7. According to Aaron Jorbin, on the Make WordPress Core blog post, WordPress r32990 introduces a change so that all classes use the PHP 5 style constructors, while still retaining the PHP 4 style constructors for backwards compatibility.
A deprecated_constructor warning that follows the same rules as deprecated_function will also be displayed for WordPress classes that are not external libraries
Chris Christoff, who contributes to WordPress core, generated a list of plugins on the WordPress plugin directory that have widgets calling
The list includes more than 4,000 plugins and contains the author, title, and slug. Plugin authors should check the list to see if your plugin is listed. Even if it’s not, you’re still encouraged to make sure you’re not using a PHP 4 style constructor in your code.
If you use any of the plugins listed, please create a support forum thread with a link to the Make WordPress Core blog post and politely ask them to update their plugin.