The Systems group at ETH Zürich where I studied had their own operating system, called Barrelfish because apparently making an OS is as easy as shooting fish in a barrel to these crazy people (this is meant positively, I hold them in high esteem). Side note they also made their own computer called Enzian. The combination of both is intended to allow them to do research off the beaten path with some different core design choices.
And we built our own student versions of barrelfish-like OSes during a course, if I recall correctly we only used their boot code to get the ARM cores on the Pandaboards up and running, then everything else was individual per group of four. We all had a lot of fun with our very individual memory management bugs, filesystem bugs, shell bugs, capability bugs and so on :-)
PS: There is also Redox OS where some people wrote an OS completely in Rust.
Agreed, and since we’re at it, same for compilers honestly.