I'm maintaining a non-public mirror of some thousand OSS repositories. When you git clone, you do not use any "default branch", you clone the HEAD of the remote repository. This is a so-called "symbolic ref". People call it "default branch" because they don't know it can also point to a tag, a commit or be in the middle of an broken rebase. And you cannot set that information remotely via git. 'git push' can't do it, as it doesn't make sense for a bare repository.
So, if you have a git mirror in a private location that you need to push to, you get royally fucked in the arse by every project deciding to change their value for HEAD. These people basically broke 'git push' for my usecase.
So either they fix 'git push' to update the symrefs as well or shut up about using anything else than 'master'.