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Updated February 2016: Vendors and details on the support have been announced: Drupal 6 Long Term Support (D6 LTS) vendors announcement
On February 13, 2008, Drupal 6 was released. The policy of the community is to support only the current and previous stable versions. (When Drupal 6 was released, Drupal 4.7.x was marked unsupported. When Drupal 7 came out, Drupal 5.x was marked unsupported.) This policy was created to prevent core and module maintainers from having to maintain more than 2 active major versions of Drupal.
With the coming Drupal 8 release, this policy has been questioned. We want to ensure that sites that wish to move from Drupal 6 to Drupal 8 have a supported window within which to do so. The Drupal core team, key module maintainers, and representatives of the Drupal security team met at Drupalcon Austin to discuss this as an in-person follow up to the previous discussion at https://drupal.org/node/2136029.
Drupal 6 core and modules will transition to unsupported status three months after Drupal 8 is released. "Unsupported status" means the community will not be providing support or patches in the same way we do now. Continuing to support Drupal 6 would be difficult for many reasons, including a lack of automated test coverage, the requirement for rigorous manual release testing, the slow-down it introduces in the release of security fixes for the vast majority of Drupal users (on version 7+), and the general shift of volunteers in the community moving their attention onto Drupal 8 development.
This gives Drupal 6 users a few options:
1) Upgrade to Drupal 7 any time between now and 3 months after Drupal 8.0.0 is released. Drupal 7 releases undergo almost 40,000 automated tests, and Drupal 7 will be fully supported at least until Drupal 9 comes out. Given the past history, the release of Drupal 9 is likely to be around 2018.
2) Upgrade to Drupal 8 after it is released, but before Drupal 6 is not supported anymore. Fortunately, Migrate support for Drupal 6 to Drupal 8 is already in core, and there is Migrate UI, a contributed module. While not all contributed modules will be ready at the time Drupal 8 is released, Drupal 8's migration path handles most of the critical site data via its CCK to Entities/Fields in Core migrations.
3) Find an organization that will provide extended support for Drupal 6. The Drupal Security Team will provide a method for companies and/or individuals to work together in the private security issue queue to continue developing updates, and will provide a reasonable amount of time for companies to provide patches to Drupal 6 security issues that also affect Drupal 7 or Drupal 8. The security team will coordinate access to issues for companies wishing to provide extended support for Drupal 6. However, the team will not explicitly review or test the patches (some team members may do this on their own). All code created by these vendors, would be released to the community.
Organizations and individuals interested in providing this level of support for their customers
AND who have the technical knowledge to maintain a Drupal core release should go to the security team Drupal 6 long term support page.
Both the Security Team and Drupal core leadership feel that a 3-month window after Drupal 8's release before eclipsing community support for Drupal 6 is a workable compromise between leaving Drupal 6 sites on an unsupported version the second Drupal 8 comes out, and acknowledging that our community's volunteer resources are limited and have shifted focus. We hope that organizations that rely on Drupal 6 will step up to help maintain it after community support winds down, and/or help their clients update to D8.