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Microkernels still need syscalls for usermode to interface with, so it is just as important. Kernels exist to serve userspace, not the other way around. Though, given the relative immaturity of Redox I assume this is less important compared to Linux.

i think Redox explicitly tries to limit the syscall exposure as well relative to linux. (dozens vs hundreds)

https://doc.redox-os.org/book/introduction/how_redox_compare...

I've never programmed a microkernel OS (and I've only done minimal Linux kernel programming), but I would assume it's even _more_ important, as a single change would impact almost every component of the system.

Does it need to? This is the rule for Linux development. Other kernels need not follow it if it doesn’t make sense.

Yes, exactly. Linus does things his way. Others (OpenBSD and maybe other BSDs for example) view the kernel + userland as a complete release and you upgrade everything together.