Drupal 9 was released June 3, 2020

This is the easiest major Drupal upgrade in a decade! Most of the upgrade steps involve making your Drupal 8 site all up to date with latest Drupal core and contributed projects. Thousands of contributed projects already moved to use the latest Drupal 8 APIs (which are still included in Drupal 9) and removed deprecated APIs (which were removed from Drupal 9). Dries Buytaert showed how the complete process looks like in his State of Drupal keynote at DrupalCon Amsterdam 2019. Here is the Drupal 9 part:

Check project status

Acquia tools for Drupal 9

Steps to update your Drupal 8 site to Drupal 9

  • Install Upgrade status on your development site.
  • Use the Administer >> Reports >> Upgrade Status page to check whether your environment is compatible with Drupal 9. (Make sure that not only your developer environment but also your live environment is compatible.)
  • Check further parts of the report for which contributed projects to remove or update. 
  • Collaborate with maintainers of contributed projects that (potentially) have issues on drupal.org still in progress, as suggested by the report.
  • Scan custom projects for deprecation issues from the report.
  • As suggested by the report then, use drupal-rector or Upgrade rector to automatically fix some required code changes (manual review still needed).
  • Repeat these steps all on your Drupal 8 site until the report shows that your projects are ready for Drupal 9.
  • Now update your core version to Drupal 9.