[djur]

`mount -t reiserfs` will also fail if ReiserFS isn't built for the running kernel. mount(2) fails with ENODEV and prints an accurate message: "unknown filesystem type 'reiserfs'". This isn't breaking userspace. The syscall and program do the correct thing based on the capabilities of the current kernel.

In any case, the ship has long since sailed on removing filesystems from the kernel. The original extfs is no longer available, nor is xiafs, nor are some virtual filesystems like devfs, etc. "A command that used to do something no longer does because the feature is no longer in the kernel" is commonplace in Linux history and is not considered "breaking userspace".