But with every build system I work with (which are: Jenkins, Concourse, Github Actions, and Gitlab CI) you can make any branch you want the branch-to-build-on.
I don't mean to say that it's not totally fixable. Up until this change, it was a reasonable assumption for any org to make, that the default branch will be the same for all projects. Now, either the default branch on any new repo must be manually set to the old default, or the build system must be updated to handle non homogenous default branches.
But with every build system I work with (which are: Jenkins, Concourse, Github Actions, and Gitlab CI) you can make any branch you want the branch-to-build-on.