Huh. I had forgotten that git does actually create a file with the branch name. But it doesn't actually screw up the .git folder or lose your data when you try to do a rename like this; it just rejects the rename unless you also use the "force" option. This has been the case since at least January of 2020. But apparently it actually doesn't always use a local file for branch names, so sometimes there's a problem and sometimes there isn't, which I guess is arguably worse than just having consistently-surprising behavior.