But is he going to find one that follows his "all has to be immutable" approach? I somewhat doubt it. Do you know of any existing OS that works like what the author describes?

The reason I posted the idea was to start with a system that is working and where you can forego the filesystem and other parts that are typically found on a full blown OS. It's not uncommon for an RTOS to dismiss the GC and leave it to the programmer.

For example, there are discussions in the RTOS world to take a closer look at Rust (Google embedded Rust) which is why the terms "new OS", "GC" and "experimental" triggered my response to look over there for a starting point.

That is a very good idea. So far I've only been able to figure out EFI booting basics.. Which is the least interesting part. Do you have any concrete suggestions?