The GitHub migration will help us better track and manage issues and will make our work even more efficient in terms of handling bugs and new features. Our goal is to simplify the development workflow by using a system that is already known to most of our contributors.
The former tracking system will still be available in the read-only mode. All issues reported in the past will still be available publicly and can be referenced. We decided not to transfer all the tickets to GitHub, as many of them are not reproducible anymore or simply no longer requested by the community. If the issue you are interested in can be still reproduced in the latest version of CKEditor, feel free to report it again on GitHub.
At the same time please note that issues reported on http://dev.ckeditor.com are still taken into consideration when picking up candidates for next milestones.
As of now please report any new issues at https://github.com/ckeditor/ckeditor-dev/issues and follow the instructions in the issue template.