Deprecation checking and correction tools to prepare for a new Drupal major version
There are several automated tools to find and fix deprecations in your codebase.
Preparing Drupal.org projects for Drupal 9
There are certain considerations you should keep in mind when preparing drupal.org projects for Drupal 9.
Preparing for use of Twig 2 in Drupal 9
How to prepare your Drupal 8 site/project for the transition from Twig 1 to Twig 2.