hook_init was a swiss army knife hook in Drupal 7. However hook_init() has been removed in Drupal 8:
You can still implement this easily enough by registering an event subscriber. However it would be nice if you still use hook_init!
This module simply reimplements hook_init for Drupal 8 by implementing an event subscriber for you and then calling all hook_init implementations, meaning you can continue to use hook_init as per Drupal 7.