Twenty Twenty-One Blocks is the name of the official Full Site Editing version of the upcoming default theme.
Twenty Twenty-One Blocks is an experimental theme created as an example to highlight what is possible with Full Site Editing.
The theme will need Gutenberg and the Full Site Editing experiment to be enabled. It will not be part of Core, but once complete it will be available in the theme directory.
During the theme creation process, the goal is also to help identify problems and limitations with Full Site Editing.
It is a unique opportunity to help shape and improve WordPress themes and the theme creation process.
The theme is part of the theme experiments repository on GitHub.
The development of the theme started this week, and the target release date is December 2020.
Everyone is welcome to contribute to the theme by creating issues or pull requests to the theme experiments repository, and take part in the discussions.
Discussions and bug scrubs for Twenty Twenty-One Blocks will take place during the Block based theme meetings, twice monthly at Wednesday @ 16:00 UTC in the themereview slack channel. The next meeting is scheduled for Nov 4th.