Obsolete branch occurs, when dev doesn’t delete branch and forgets about it. Obviously simplest obsolete branches to remove are the merged branches. Those doesn’t contain any commits, that would be missing on develop or master branch. Unmerged branches contain extra commits both in repository and on author’s local. If author doesn’t care about a branch for long then the commits are probably not very valuable. Thus repository should not be burdened with such obsolete branches.
Using above reasoning I’ve provided git-obsolete-branch-removal to automatically remove obsolete branches. And we were happy ever after.