For now, you can try to compile it from source and then register this php handler in Plesk.
Note: PHP 5.6 will stop receiving security updates after 31st of December 2018. For more information about the PHP Lifecycle, please click here.
Plesk does not support any software being EOL(End Of Life). Using this version and all related dependencies are at your own risk. Moreover, extension management from Plesk > Tools&Settings > PHP Settings > PHP 5.6 won't be available for this version. As a workaround, manual setup of php.ini file has to be performed.