I reinstalled everything from scratch and the only way to add disable functions for 1 user is the below quoted steps with PHP-FPM.
I checked again the PHP configuration settings via phpinfo() function and I saw again that the path of PHP Loaded Configuration File is /opt/alt/php-fpm74/usr/php/php.ini and not /home/user/public_html/php.ini that means the extra new disable_functions record can't be read from the user's custom php.ini file.
I don't know what else to do to have the path of PHP Loaded Configuration File on user home folder (/home/user/public_html/) that the custom php.ini file exist.
1. CWP Admin, Webserver Settings, Webservers Conf Editor, PHP-FPM
2. php-fpm74 tab
3. Edit a user account conf
4. Add to the bottom: php_admin_value[disable_functions] = shell, shell_exec, system
5. Restart/Reload Apache and PHP-FPM