question: Is there any classic operating system known
for which the
complete source code is available ?
CP/M. You can download the source, but it's not "Free Software". You have
to get a license from Caldera for commercial use.
If you don't mind something a bit esoteric, it *might* be possible to
convince the people that wrote Apex to release the source. Apex was only
"marketed" for the Apple ][, although it was ported to other systems including
the Sage/Stride 68000 boxes and even a few models of Macintosh.