By default, the function shell_exec is available for customers. If this function is not listed in the disable_functions parameter value field, it is enabled.
This can be checked in Plesk > Domains > example.com > PHP Settings > View phpinfo() page, as described in this article.
However, FFmpeg is not shipped by default within Plesk, but it can be installed for PHP 5.6 provided by Plesk only. It cannot be installed for PHP 7.x provided by Plesk due to conflicts with Zend, as per this article.