Plan 9 had Venti, which was indeed a content-addressable file system. But it was not the main file system for day-to-day use; it was intended to store backups

Plan 9 had Venti, which was indeed a content-addressable file system. But it was not the main file system for day-to-day use; it was intended to store backups only. Btrfs is not content-addressable, although everything else you say about it is correct.