I'm not an expert on this package nor this situation, however, looking
at the htmlize.el file shows:
;; Copyright (C) 1997-2003,2005,2006,2009,2011,2012,2014,2017,2018
So it would appear that the package has existed longer than either
GitHub or Org. I'm guessing that its author moved its primary repo to
GitHub after Org started using it.
In hindsight, perhaps it would have been better to move htmlize into
Emacs before adding it to Org proper. But many things are clearer in
hindsight.
Regardless of where it is hosted, htmlize.el is Free Software, and it's
an optional addon to Org. We can encourage its author to add it to
Emacs proper, or to ELPA. But surely it's not necessary to censor the
mention of "GitHub" in the manual; it's simply a fact that GitHub exists
and that htmlize.el is currently hosted there. It would seem
unreasonable for the Org maintainers to have reacted to htmlize.el's
moving to GitHub by removing htmlize.el support while it remains Free
Software, yet that's the logical conclusion of this argument.
So please do not remove support for this package because of where its
repo happens to be hosted at the moment. That would be a major
regression, and it would not be in users' best interests. It would not
be fair to remove a major feature used by thousands of users and demand
that "someone" (since there is no one ultimately responsible) rewrite
large parts of ox-html.el to fix it. It would at least seem fair for
those insisting on the change to do the necessary work.
