PHP has deprecated the mcrypt extension in PHP 7.1, and will remove it entirely in 7.2. The extension is also often not available in hosted environments, regardless of PHP version.

However it is still shown as required by the Joomla installer, and used by several parts of Joomla.

Given that PHP 7.2 is due out imminently, we should probably be looking to remove this dependency.

(this issue may have been raised already; if it's a dup, please close, but I couldn't find it as searching issues for mcrypt pulls up loads of unrelated tickets where people have referenced it when describing their configuration)