The C compiler source came with the AT&T UNIX tape ... it wasn't open source because FOSS didn't exist at the time and it was under a proprietary license, but it was not "closed source". Parts of MOJO, OTOH, are closed source because the source has not been released (yet ... it apparently will be later this month).

That one yes, the ones sold by all UNIX clone vendors definitely not.

In fact, GCC was largely ignored until Sun decided to split SunOS into two SKUs.