How can I find all the listeners or subscribers to the KernelEvents:REQUEST event? More specifically, how could I find the subscribers performing the routing system for Drupal? 

We tried using ddebug_backtrace(), but I couldn't get anything.
Instead of ddebug use the Symfony Webprofiler, that is included in the D8 version of the module Devel:

https://www.drupal.org/project/devel

WebProfiler (D8+)

Add a powerful footer to all pages of your site. There, admins can review resource utilization, cache effectiveness, database queries, Views, and so much more.