slcurses is supposed to be a replacement for ncurses, so why does slang-2.1.1 depend on ncurses? No ncurses library is pulled in by an ebuild and no installed library or executable depends on ncurses. So, unless I'm missing something, the dependency on sys-libs/ncurses is not needed and can be safely removed.
Completely doesn't matter, sys-libs/ncurses is part of system, you cannot unmerge it (see $PORTDIR/profiles/base/packages)