A couple of developers I spoke with during DrupalCon Prague expressed concern about the disappearance of the Source button. It will become impossible to use HTML editing because of this. In my experience, HTML editing is used primarily to find workarounds for editing problems. Hopefully, in CKEditor 5, we won’t need these workarounds anymore.
The update to CKEditor 5 is a big deal, as one can tell by the number of sessions during DrupalCon Prague dedicated to this project. CKEditor 5 has been written from scratch. It is an entirely new editor with different architecture and APIs. The new architect makes browser-specific bugs a thing of the past.
The current way the issue queue is organized can be pretty frustrating for module maintainers. There is little room for project management or prioritizing issues. To solve this, the community created ContribKanban. With GitLab, each project gets Kanban boards out of the box to help manage the issues.