So far "master" has been used. With the parallel development of KF5 & KF6 after branching, at least one new name has to be found, either for the KF5 one or the KF6 one.
One idea: using "main" for the KF6 branch.
This would allow to start picking up that branch name that has become the usual branch in some other developer bubbles, when a new name has to be introduced anyway, with all the adaption work needed. "main" as branch name for Qt6/KF6 code variant might be also used in non-KF repos then, for consistent branch naming across KDE repos.