I heard "copywritten" on a Madeline Brand article on NPR. If it was on NPR, is must be OK, yes ?
More seriously, and in the pedantic tradition, here's the right way to speak about copyright:
One does not "copyright" a work; one "claims copyright in" a work. If one files the paperwork with the Library of Congress, one "registers a claim of copyright in" the work. A work in which copyright is claimed is, however, ordinarily spoken of as "copyrighted", even sometimes by judges. But true pedants will say that "copyright is claimed" in it, and speak of it not as a "copyrighted" work, but "a work in which copyright is claimed."
I hope this helps, or at least amuses.