While the latest version of Kayako Classic is 4.99.8 we recommend running the latest stable version 4.98.9.
4.99.x was brought to life to support PHP 8.2.1 as can be seen in Kayako Classic Server Configuration and System Requirements but this proved to be more complex and resource-intensive than anticipated and the multiple 4.99.x versions were deemed unstable. You can refer for example to "Cannot access protected property" PHP Error After Upgrading to Version 4.99.8
At the same time, Kayako Classic reached its End of Engineering milestone. This means that no other versions will be released or existing defects addressed.
Therefore we recommend our customers install and use the latest stable version 4.98.9 which supports PHP 7.3.x
Make sure you check Kayako Classic Server Configuration and System Requirements and install it following the instructions in Installing Kayako Classic Download
About the End of Engineering
As explained the code of Kayako Classic will remain unchanged from now one and won't receive more versions or bug fixes.
You will be able to continue using Kayako Classic unencumbered and add user licenses as necessary, and our support team will continue to respond to issues and use reasonable efforts to assist you.
Alternatively, if you need a particular enhancement that is not available we would like to recommend switching to our New Kayako (Cloud) product where we continue adding new features, improvements and bug fixes.
Kayako Classic has reached the end of its engineering life. While the latest version of Kayako Classic is 4.99.8 (PHP 8.2.1) we recommend running the latest stable version 4.98.9. using PHP 7.3.x and MySQLversion 5.6.31 or MariaDB 10+