The package must deliver its full functionality and convenience
on a
completely free platform based on the GNU operating system (in
practice, GNU/Linux), working exclusively with other free
software.
Otherwise, it would act as an inducement to install nonfree
systems
or other nonfree software, and that would work against our
cause.
However, as an exception it is ok for a package to provide, on
some
non-GNU operating systems, features that the rest of Emacs (plus
GNU
ELPA and NonGNU ELPA) already supports on GNU.
This is a moral issue. See https://www.gnu.org/prep/standards/,
node System Portability. The reason for this rule is that at no
time, in no way, should a NonGNU ELPA package put users who
defend
their freedom at a disadvantage compared with those who
surrender
their freedom.