Well, that was especially true for Minix 1 and 2. Minix 3 is purportedly a production operating system, and ships in every Intel CPU for the last decade or so, right? So while admittedly it may not have significant filesystem needs, it is not purely a teaching operating system anymore.

Nowadays I might point at MIT's xv6, which implements a very similar simple unix filesystem: https://github.com/mit-pdos/xv6-public/blob/master/fs.c