While Drupal 9 already comes with CKEditor 4, that will go end of life in 2023, so we need to upgrade to CKEditor 5 to provide this replacement for Drupal 10. The target release date for Drupal 10 in June 2022 (in 15 months!). We plan to add CKEditor 5 integration to Drupal 9 even sooner though, to help the Drupal contributed ecosystem catch up and prepare in time.
There are various moving parts and several items left on the roadmap for beta level core inclusion. Peter presented a demo today to showcase where the current state stands primarily to get more feedback about the developer interface. A simple infobox CKEditor 5 plugin was also showcased to asses the integration developer experience. Check out the video recording here:
Discuss in the #ckeditor5 channel on Drupal Slack. CKEditor 5 integration meetings are every other Thursday at 15:30 UTC in the same channel.