I think the only SoC that's close is the one that comes on the Raspberry Pi 1-3, since they even got broadcom to release the docs for the VideoCore IV GPU.

It's still not completely documented, but it's better than most "Here's the docs for the basic I/O peripherals but we only provide closed drivers for Linux" SoCs.

Otherwise there's still just the regular old x86 PC everyone has lurking in their modern PC.

And that will still very much resemble most of today's OSes not only because that's what people know, but also because the development kit will need to run on common OSes, so the paradigms still have to be somewhat compatible.

There's a lot going on, actually.

I wouldn't dare call seL4, haiku, genode or managarm toys.

There's SerenityOS -- https://serenityos.org/ One of the main authors has been documenting the development on YouTube, it's pretty fascinating.