In his State of Drupal keynote at DrupalCon Amsterdam, Dries Buytaert showed once again some tools to use to prepare for Drupal 9 including the Upgrade Status module. To me the process is even more interesting than the tools, because it is entirely different from the last upgrade. As I wrote last week, you now make a lot of incremental improvements on your existing Drupal 8 site that makes the gap to Drupal 9 significantly smaller.
It is a new mindset to look at your Drupal 8 site to improve in preparation for Drupal 9 and we have tools to identify those improvements to make. However Dries also mentioned that we are not quite there to automate those improvements. Back in May Dezső Biczó of Pronovix built out a proof of concept integration with Rector that implements a sample set of refactors, including changes for global constants and best effort solutions to some EntityManager uses, a UrlGeneratorTrait and a drupal_set_message() rector. While the extent of impact of the global constant rectors are not yet known due to limitations in our tools not finding them yet, the rest of the implemented rectors are definitely tapping into the top list of deprecated APIs.
Unfortunately slightly after he posted his blog post about the proof of concept, Dezső did not have time for this project anymore. I think this tool could be of huge help in removing deprecated API use in your projects and thus making them more modern Drupal 8 projects (while being more if not already entirely compatible with Drupal 9). However, we need contributors to cover more of the deprecated APIs, especially around file_*() and db_*() functions. If we can figure out rectors for most of the top 15 errors (and Dezső already did some of them), we could cover over half of all deprecated API use in contributed projects:
To top that off, I also think a simplytest.me style online service would be very useful to run drupal8-rector on your drupal.org project and suggest a patch to apply to remove deprecated APIs. Even better if it allows custom code to be uploaded so people can use it for that too. The more we can automate these tasks the easier the transition of Drupal 8 to 9 will be.
Submit issues and pull requests in the drupal8-rector project to improve API coverage. Look for me on Drupal slack if you get stuck and I'll try to help. I'd also love to talk to you if you want to set up an automated service. Let's do this!