In Drupal 8.4.X and later releases, Drush 9 is the only supported and recommended version. One of the key changes introduced in this version is a new model of writing custom Drush commands. From now on, .inc files are obsolete and you will no longer use them for your commands, which are now classes based on AnnotatedCommand format.
The underlying structure of a module containing your custom Drush command will look as follows: