Is MINIX still the go to for learning about operating systems?

I don't know any courses teaching OS courses using Minix today (though there's probably one at VU Amsterdam).

Many courses use MIT's xv6 nowadays, but as a modern lookalike of 6th Edition Unix, this is a quite traditionally structured OS. I don't think there are any textbooks teaching OS design and implementation starting from a microkernel architecture.

For me, Douglas Comer's Xinu book was my first OS textbook, but in my own OS courses, I let the students develop an OS on bare-metal RISC-V from scratch.

xv6 is a terrific project to grok the basics of Unix. very readable and gives you syscalls, context switching, and kernel/user space dichotomy. I also like 3EasyPieces from the couple at Madison (UWisc) - can’t remember their name just now.

I've read Linux source code from the 2.x series. I found that to be quite doable. Then again, I was only interested in specific components like memory management. For getting the big picture, Linux may be too much.

The x86 Assembly code of MikeOS is remarkably readable https://mikeos.sourceforge.net