Can I ask what you'll find it useful for? The technology is neat, I'm just struggling to think of something that would benefit from it.

Primarily for soft (or hard) real time operations. Even kernel modules running in a RT-Linux have a difficult time responding consistently to an IRQ at sub-10 uS latency. Of course, the larger SoC capable of running Linux can have hardware latencies due to shared data buses, etc. but still having a RTOS layer “underneath” Linux would be a good way to ensure more consistent responses.

It does support arm, it's one of the primary ways to use sel4.

Does that mean you could run l4linux with sel4? Didn’t realize the l4 api is consistent/broad enough to do that. That’s cool! The linked “drops” rtos seemed to be dead a long time back.

Not quite. It was rebranded and even is commercially supported these days: https://www.kernkonzept.com/l4re.html

There is no documentation that I can find regarding Linux running on seL4, outside of hardware virtualisation.