(Contrary to what I said in that Fediverse thread, I think that V7 and earlier may not have actually had a zero inode. Since I'm the sort of person that I am, this made me wonder if you could legally use inode zero today in a POSIX compliant system. The OpenBSD stat(2) and getdents(2) manual pages are similarly silent about whether or not the inode number can be zero. The cpio archive format does, but I can't find a mention of a zero inode value having special meaning. The POSIX pax archive format is similarly silent; both cpio and pax use the inode number only as part of recording hardlinks.)