For example, on z/OS, the whole disk storage subsystem (SMS, but there is more) is separate from the MVS (kernel). Security is also externalized in RACF server (in fact there are alternate products from non-IBM vendors). You can run multiple TCP/IP stacks, which are also running in their own address spaces. Sysplex serialization has its own address space.

All the address spaces involved in the operating system are coordinated through SVC or PC routines, which are like system calls, and scheduling of SRBs, which are kinda like kernel threads. I am not sure (although I am not aware of latest developments) if in Linux one can define a custom system call, like you can on z/OS. Or if you can schedule your own kernel thread from user space.

You seem to know about MVS, yet we probably disagree on whether it is to be called a microkernel or not. I am not an OS expert, and I never did kernel-level programming for Linux or z/OS. But I did read Lister's Fundamentals of Operating Systems long time ago, and that book is somewhat based on what MVS (the actual kernel of z/OS) does. It was written before the whole microkernel debate, which AFAICT might be just an artifact of enormous variety and complexity of x86 hardware.

So I would like to hear, in your opinion, what should have been different in MVS (or z/OS) for you to consider it a microkernel?