Mach kernel is open source for much longer time than that term itself exists. Original "GNU system" (what is known today as GNU/Hurd) was supposed to use Mach as kernel.

In late 80's there were multiple projects that attemped to build unix-like system on top of Mach (what essentially is academic project) out of these NeXT/OS X is the only one still really relevant. Another such project was OSF/1, which also took the way of running essentially all of real unix kernel space as one big Mach thread with code derived from BSD, this became DEC/Compaq/HP(E) Tru64 and all other systems build on this OSF/1 base never really worked with sufficient performance. (And for Tru64 on it's original Alpha platform Mach is slightly weird choice as the CPU essentially comes with it's own "microkernel" in microcode/firmware).

Yes I mis-remembered. The source was always open, but parts of it were encumbered by ATT Unix licensing.

It was heavily based on 4.3BSD, so it needed an AT&T 32v license at the time....

https://unix.superglobalmegacorp.com/cgi-bin/cvsweb.cgi/mach...