Is hook_boot() removed from drupal 8?
You can see here


  In Drupal 8 hook_boot() was removed since the same results can be
  achieved with other ways, for example:
  
  
  A module that needs to interrupt the request very early based on certain conditions can use an event listener (see #1794754: Convert
  ban_boot() to an event listener) (Note that modules are loaded by this
  stage unlike in Drupal 7.)
  A module that needs to run on cached pages should prompt its users to add code in settings.php