The mcrypt extension was deprecated with PHP 7.1 and was removed from PHP 7.2 and moved to an unofficial PECL extension because the mcrypt library is no longer maintained.
For PHP 7.2+, PHP instead uses sodium as a cryptography library, because of this new PHP code should be written to use sodium rather than mcrypt. Acquia recommends against using the mcrypt library over the sodium library. Acquia cloud now fully supports sodium, as seen in our change-log: