As of cPanel & WHM version 74, cPanel will begin to deprecate their support of SquirrelMail, one of the bundled webmail applications in cPanel. cPanel expects to stop shipping SquirrelMail for new installations of cPanel & WHM in version 76 and will remove support of it with version 78. As this change will disrupt many users, cPanel is taking this opportunity to explain the reasons behind their decision in the below URL. cPanel also is opening a dialogue with you, their community, about their next steps.
Please note that we are now about to switch to cPanel version 70 in the stable tree (which will be introduced around Q2 2018 to the stable repository), it is however important that users which are currently using squirrelmail will need to start looking at an alternative.
cPanel keeps supporting Horde and Roundcube for now. We as 040Hosting will no longer support squirrelmail at the moment cPanel stops supporting it as well.