Drupal 8's last minor release = Drupal 9?
Yes, upgrading from Drupal 8 to 9 is scheduled to be an incremental update from the last minor version of Drupal 8. 

It seems the main thing to consider is to remove any usage of deprecated functions to ensure a smooth upgrade. 


  Contributed project developers, as well as those maintaining custom integrations, should follow the deprecations when possible and use the latest APIs available. This means that when Drupal 9 is released they will have to make fewer changes to be compatible


https://www.drupal.org/core/deprecation