Gábor Hojtsy on 27th April 2022 through a blog post announced that Drupal 10 will be released on 14th December 2022. This gives them more time to stabilize CKEditor. It also means more time for site owners to test moving their content from CKEditor 4 to CKEditor 5 in Drupal 9, so that there is a smooth and safe upgrade path for this major change.
The Drupal Association had earlier announced that they were considering three possible windows to release Drupal 10. But, they came across some critical issues that needed to be solved in order for CKEditor 5 to be stable, and these issues won't be completed in time for the 13th May beta deadline required for the August release.
The most critical requirement for Drupal 10 is our CKEditor 5 integration. CKEditor 4 is end-of-life at the end of 2023, so Drupal 10 must use CKEditor 5 instead. Drupal Association has collaborated closely with the CKEditor team and spent thousands of hours working on Drupal's CKEditor 5 integration.
The December release also means they can release Drupal 10 with Symfony 6.2, which will have improvements and bug fixes over the current 6.0 version, and will also reduce the workload for the Security Team.
Drupal 10 will require PHP 8.1, and the December release means that most hosting service providers will support PHP 8.1, so that sites don't have to wait for platform fixes to start their Drupal 10 upgrades. PHP 8.2 is also scheduled for release in November, and Drupal 10 will include as much forward-compatibility with it as possible.
Under the December schedule, Drupal 9.4.0 will be released on 15th June. Drupal 9 and 10 development will continue in 9.5.x and 10.0.x after that date. All requirements for Drupal 10 must be completed by the beta deadline of 9th September, 2022. In the week of 12th September, beta versions of both Dupal 9.5 and Drupal 10 will be released, and the stabilization and testing phase will begin.
Drupal 9 is end-of-life in November 2023, because that is the end of life for both Symfony 4 and CKEditor 4, which are dependencies of Drupal 9. This means site owners will only have 11 months to upgrade their sites from Drupal 9 to Drupal 10.