Comment 1 for bug 1959968

You need to set the new default branch in the Launchpad UI.

- go to your repository
- at the right side click on "Change repository details"
- rename e.g. refs/heads/master to refs/heads/main

Please make sure that open merge proposals get merged first, or you will need to resubmit them.

I agree that there should be a proper help document for this.