FWIW unlike HN, downvoting has no negative consequences to the comment on GitHub. It's useful on HN to effectively remove the off-topic and deeply non-constructive comments. But on GitHub, it serves no such function. We're going to end up with the many abuses of -1's and none of the benefits.

As for "I don’t think it’s 100% clear-cut that you should never ever implement downvotes," perhaps. There's still probably better ways to flag inappropriate comments though. Like a flag button. Downvotes are far too general and ambiguous in comparison.