NOTE: There won't be a pull request for every single change; some changes may be committed directly to the main branch. See the commit history to keep up with the changes.
Here is a list of items marked TODO.
This plugin aims to bring TinyMCE version 5.x to the ClassicPress editor which is currently 4.8.0.
TinyMCE is used on the post editor page, the Text Widget, and anywhere a plugin adds it (like metaboxes or taxonomy descriptions or user biography).
The Code Reference shows only the PHP side of things, which includes using quicktags.js also. The Text Widget (and perhaps some plugins) sets it up from Javascript (see wp-admin/js/text-widget.js).
Please install the plugin and give it a try. Don't be gentle. Put it through the paces. Try weird things. Actively try to break it. And then report your findings here! The changes mentioned in the migration document are of particular interest.