Created in 2003 by Frederico Caldeira Knabben as FCKEditor, a JavaScript based WYSIWYG / Rich Text editor for web applications.

CKEditor 4 was released in December of 2012 (Drupal 7 was released in 2011)

Drupal 8 (released in November of 2015) integrated CKEditor 4 at the core level

CKEditor 5 was released in 2018; EOL for Version 4 set for end of 2022

Drupal core team actively working to integrate CKEditor 5 as part of the Drupal 10 release.

This has pushed off Drupal 10's release from this June all the way to December.