The Asterisk 21 branch has been created! This is the first step in the process to seeing the first release candidate created followed by the actual release. This is currently slated for October. For specific dates you can always consult the Asterisk Versions page on the documentation site.
From a user perspective there’s nothing for you to do yet, but you could use the Asterisk 21 branch if you wish and report any issues.
From a developer perspective you must now remember to also specify cherry picking to the 21 branch when submitting a pull request to ensure your change also goes into that branch.
Just a reminder as well that both Asterisk 16 and 19 will lose security fix only status this year. It is recommended to upgrade to the latest LTS version, 20, if you are still using either.