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Drupal 7 and Drupal 8: End-of-Life
As of November 2022, both Drupal 7 and Drupal 8 will reach end-of-life status, which means updates will no longer be available to either. By announcing the release of Drupal 9 over a year before it happens, and two and a half years before the older versions are no longer part of the developing cycle, the people at Drupal are giving users enough time to fully prepare for the migration, with enough warning to be fully upgraded by that time.
The wide release window also provides core contributors with time to work on their modules and adjust to the upcoming changes, not to mention that it gives the Drupal team time to iron out any kinks.
What About Symfony?
As for Symfony 5, it will be shipped with Drupal 9, but it will also be compatible with Drupal 8 while the developers identify and fix any potential issues, giving users enough buffer before S5 is required for D9.
As of yet, not much is known about which third party dependencies will be updated, or even which modules will survive or be removed.
The most important thing to remember about Drupal 9 is that it is not really about new features, which developers have claimed will be few if any at all. The entire point of the upgrade is to ensure a general migration from older versions, which will not have any security updates going forward past 2021.
If you or your company use Drupal, be sure to keep checking the O8 blog for more updates on this release as they become available.