I'd like to see more information about that. I remember that Penguin Computing offered such some 15 years ago, but don't know where it was deployed or still is. Cray and IBM had also such a concept for their superclusers in the past, but are they still using such? The one HPC environment I worked on (a major car manufacturer in Europe) used plain RH Linux on all nodes as recently as three years ago.
The current #1 (Fugaku) uses IHK/McKernel as kernel for the actual payload. The previous #1 (IBM Summit) seems to use RH Linux though. Perhaps, since the most performance critical part is run by and within the GPGPU(s), the actual OS doesn't matter all that much (for performance -- it matters of course for programmer's comfort/efficiency).
There used to be a lot of "special microkernel on compute RPCing to Linux on I/O" on Crays and the like. Hard to say how prevalent it is now, and most annoyingly I can't recall the names. (Charon?)
