It’s certainly been interesting watching the multi-decade arc play out. With Mach as the origins, everything other than tasks (processes) schedule and virtual mem was out of kernel and done over Mach port comms. Then xnu via next step and later OS X linked much more in kernel and exposed specific data types using com+ in iokit. And now more and more is moving back out of the kernel.

io_urging networking on Linux is another similar move out to use space