I completely agree to the simple answer and to the deeply nuanced one… 

And I just wanted to add my two cents: System 7.x cannot be considered abandonware in any way, since it is still published and distributed online by Apple, though free of charge. The same goes with System 6.x.

II2II could have added a point d: because the software is no longer distributed officially by its publisher. I would consider System 1.x to 5.x as abandonware, because you can't find those on apple.com, for example.