cPanel made an announcement on their official blog about deprecation of Squirrelmail as a webmail option in cPanel.
As of cPanel & WHM version 74, cpanel will begin to deprecate support of SquirrelMail and will stop SquirrelMail for new installations of cPanel & WHM in version 76 followed by removing their support with version 78.
Why SquirrelMail is being removed ?
SquirrelMail’s last update was May 30th, 2013, with their last release on July 12, 2011. In that seven years, 4 versions of PHP have reached End of Life. Unfortunately, recent security patches have significant problems, forcing cPanel into a choice. Exclude the security fix and ship SquirrelMail with known security flaws, ship a secure version with known interface issues, or attempt to fix the problems.
Webmail users now expect a better experience than SquirrelMail provides. Rather than continuing to ship an unmaintained application, so cPanel decided to remove SquirrelMail.