I'm certain that I'm not thinking of somewhere else. I'm completely open to the possibility that I just remember it wrong, but I'm sure that it was HN that I was thinking of, and not another site.
There's never been such a policy on HN; you remembered it wrong, as have many before you. It's the same phenomenon as attributes pithy quotes to Einstein and makes Canadians think we have Miranda rights: people hang memories on the nearest pre-existing hook in the brain.
Nope. I've never spend much time on reddit, and I'm certain I've never seen that page before. Perhaps I am thinking of comments other people made on HN in the past (who thought that the policy was as I described).
Nope, I know I'm not thinking of Reddit (as I said in reply to another comment, I've spent almost no time on Reddit, and would not have seen their guidelines at all).
Probably the most that I've written about downvotes on HN is at https://news.ycombinator.com/item?id=9440694.