The documentation says master: https://git-scm.com/docs/git-init#Documentation/git-init.txt... but "may be subject to change"
When running "git init" on a fresh install, you also get a hint that the branch name may change. So I think they are still working on changing it to main upstream: https://lore.kernel.org/git/pull.762.git.1605221038.gitgitga...