Github and bitbucket (and likely others) have the concept of a default branch.
This nominally is "master" by convention with git, but it is common enough for people to make up their own convention. With github you can tell github what the master branch is, and this changes the behavior of the UI somewhat. This is also exposed via the api.
Given the importance of main/master/default branches in UI, it would be nice to be able to expose this via the respective plugins. The common dependency I believe is scm-api (even though the git implementation won't implement this extension, one of the github ones can, as it depends on git, which depends on scm-api)