The merging of Mach with UNIX was a mistake given it was one of the slowest, over-complicated microkernels. Far as I'm aware, they'd prefer to remove Mach but it's a big job at this point that might affect their ecosystem a lot. If you are interested in open microkernels, I encourage you to look into L4 family (eg OCL4 or OKL4) since they're far more advanced than Mach-based designs. Minix 3 does one with NetBSD userland and self-healing capabilities. GenodeOS is turning one into a desktop. IBM's K42 ran on NUMA machines. JX OS mixes one with a JVM to run the OS in Java. For security, GEMSOS was a security kernel w/ MAC. KeyKOS and EROS were examples of those doing capability-based security. Modern ones like Muen and seL4 just do separation w/ apps managing security policy.

EDIT: Windows is also a "semi-microkernel" with a microkernel inside of it on the bottom that seems to be used for consistent way of interfacing components more than anything. I'm not sure if it's still in there.