Updated on 18 October 2017
Drupal is based on semantic versioning for its development release cycle.
What will be the difference between Drupal 8.x and 9.x?
Each new 8.x release introduces API changes with backward compatibility.
API’s that were deprecated will be removed in Drupal 9.
So, if your custom code follows the API changes the Drupal 9 update should be easy.
Resources
- Making Drupal upgrades easy forever by Dries Buytaert
- There will be a Drupal 9, and here is why by Gábor Hojtsy
- Should I Use Drupal 7 or Drupal 8? by OSTraining