I remember using the Minix filesystem on floppies. ext{,2} was a bit too heavyweight for a floppy. And UMSDOS on my MS-DOS6.22/Wfw3.11 FAT16 partition. And HPFS so I could mount my OS/2 2.0 partition (EMX on the Hobbes OS/2 CD-ROM was my gateway drug to Linux.)

xiafs sounded intriguing, but I never actually used it.

Of all the filesystems currently in the mainline Linux kernel, the most useless (surely) must be BFS (not BeFS) – https://www.kernel.org/doc/html/next/filesystems/bfs.html

Hey, I support a system that 4 complete rewrites ago used BFS!

There was also Minix, used in the very early days.

My undergrad operating systems class used Minix

Isn’t minix shipped with every x86_64 cpu these days? I thought it was one of the most deployed operating systems ever

I think Android beats MINIX.

The file system used isn't accessible, so issues aren't directly applicable.

Even today, Open BSD use FFS. You can mount ext2/3/4 and FAT but pretty sure only FFS is supported for the root file system.