Dries Buytaert recently published a great post on how to prepare for Drupal 9. He explains how we build Drupal 9 in Drupal 8 using deprecations and the tools to use to detect use of deprecated code. Acquia funded me and Zoltán Herczog to work on one of the tools in the past few weeks. Zoltán just released the second alpha of Upgrade Status. It is definitely worth a try!

Here is how it works: