A do-it-yourself fix - -

 

https://blog.cpanel.com/the-death-of-squirrelmail/

 

How to keep SquirrelMail after removal - -

The wonderful folks ("quiet cough") at CPanel said - - "..We understand that SquirrelMail’s faithful fans will not welcome this decision, but we have good news!  You can continue to use SquirrelMail after version 78 because cPanel & WHM supports custom webmail applications. Follow our documentation on Integrating Custom Webmail Applications, and get the code from the SquirrelMail website..."

 

https://documentation.cpanel.net/display/DD/Tutorial+-+Integrate+Custom+Webmail+Applications

 

https://squirrelmail.org/download.php