Gábor Hojtsy: How to automate testing whether your Drupal 8 module is incompatible with Drupal 9?

Drupal 9 is planned to be only 18 months away now, wow! It is already being built in Drupal 8 by marking APIs to be removed in Drupal 9 as deprecated and eventually upgrading some dependency version requirements where needed. Once the Drupal 9 git branch will be open, you will be able to test directly against Drupal 9. That should not stop you from assessing the compatibility of your module with Drupal 9 now. To prepare for compatibility with Drupal 9, you need to keep up with deprecated functionality and watch out for upgraded dependencies (when we know which are those exactly). Of these two, automation can go a long way to help you keep up with deprecated APIs.