MCrypt extension is abandonware and dependency on it should be removed
Per the deprecation notes for PHP7.1 - http://php.net/manual/en/migration71.deprecated.php, The MCrypt extension has not been maintained for a long time, and is actually removed from the core distribution to PECL in 7.2.
Apart from installing PECL packages not being achievable in many hosting environments, use of an abandonware encryption extension is not great from a confidence perspective. Parts of the system that are currently dependent on MCrypt ought to be retooled to use OpenSSL functions, per the PHP project's recommendations.