I'm active user of ISPmanager, I use it for multiple servers since long time, It has been long time ago since ISPmanager clients asked for adding Varnish-Cache server support, this was up-voted, included in your plans and then approved. Now it's dropped from your plans for some reasons. Varnish-Cache server is necessary for many CMS like Magento (it's a must for Magento!), Drupal and others. If you can't add Varnish support now, please consider adding it to your plans, meanwhile please provide a description of how it can be installed & configured without breaking the functionality of ISPmanager.

Best regards,