Couldn’t help but suggest the UNIX tag.
Is it not the case that all the good ideas of Hurd have been implemented now, elsewhere (aside from drivers in userspace perhaps?)?
Granted, it’s still interesting to see work still being done on it…
NT does a lot of drivers in userspace, and FUSE is very much userspace filesystems.
Microkernels are still an interesting space, and most OSes are have taken the concepts or are right on the edge of being considered one. Mach is the most popular school of microkernel design, though you have others like L4.