We recently upgraded squirrelmail to be compatible with PHP 8.0+.
The upgrade unfortunately disabled some features with respect to theme color and font choices that some of our customers preferred.
Consequently I’ve restored the old version along with configuring this application to use PHP 7.4 for which it is designed.
I’ve been trying to find some newer more modern web mail clients, we do also have raintree and roundcube but those tend to be optimized for portable devices. Squirrelmail is the oldest and least well supported of the web mail clients that we have, yet it remains by far the most popular which makes it a challenge.