symfony/http-kernel is listed under suggest but then in Logger.php it has:

use Symfony\Component\HttpKernel\Log\LoggerInterface;
use Symfony\Component\HttpKernel\Log\DebugLoggerInterface;
...
class Logger extends BaseLogger implements LoggerInterface, DebugLoggerInterface

which causes exception if symfony/http-kernel isn't available.

symfony/http-kernel needs to be added to require in composer.json or the use/implements dropped.