alt Hacker NewsI had a lost+found folder in all Unix file systems I used since the 80s. It's where fsck places files that it found during a scan and can't figure out to which directory they belong. Sometimes I found stuff in there.
From what I googled XFS, Btrfs and ZFS don't use lost+found. It's a thing of the old not journaled filesystems and of the ext family.
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Even with journaling, you might need one. ZeroFS [0] almost had a lost+found directory (even with the WAL enabled), because you might have consistency issues between your in-memory state and what was flushed, and especially in what order.
ZeroFS ended up not needing recovery at all through atomic, strictly ordered commits [1], but it was far from trivial (and not just a matter of requiring a WAL).
[0] https://github.com/Barre/ZeroFS
[1] https://github.com/Barre/ZeroFS/blob/main/zerofs/src/fs/writ...
XFS does use /lost+found, it calls it the "orphanage directory" and xfs_repair reparents children of corrupt directories there.
Based on comments in the kernel source, it seems like the userspace fsck for JFS and F2FS will also sometimes create /lost+found. There might be more that do.