With the release of PHP 8 in November 2020 numerous changes were introduced. This was a major release for PHP and breaking changes and improvements were introduced. The latest versions of PHP allow more flexibility to its users and saves them important development time.
For now, phpList cannot support these changes. This means that you might encounter issues with your phpList installation while using PHP 8, and it is advised to continue using versions prior to PHP 8. Also, have in mind that support for PHP 5 has been discontinued since 2019, and therefore you might encounter issues with your phpList installation. For now, PHP 7 is a safe option for your phpList installation.
You can always pay the community forum a visit for any installation problems you might encounter.