#Needs documentation

It appears that the use of Symfony\Component\HttpKernel\Event\FilterResponseEvent was removed in Drupal 9.0.0, and replaced with Symfony\Component\HttpKernel\Event\ResponseEvent in 9.0+.

The class, https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Http%21KernelEvent.php/class/KernelEvent/9 appears to be a port of the Symfony 5.3 KernelEvent, with some added code for forward compatibility with Symfony 6.

This class, Drupal\Core\Http\KernelEvent, was removed from Drupal core with version 10.0.0.

Since the code in question in this issue has been deprecated and removed from Drupal Core, I'm going to close this issue as outdated.