HN wants the most substantive article for a story, and also prefers non-paywalled links. From that we can deduce one bit of etiquette, which is that if you post a paywalled article you're implicitly promising that it's the best one, i.e. that you searched and there's nothing else as good on this story. In such a case, as long as it falls in the workaround-able category (which most paywalls do), it's ok.
That's on the submitter side. On the reader side, the etiquette might be that you should point out a non-paywalled URL if you can find one that's as good (in which case we'll be happy to switch to it), and also that you probably shouldn't post generic complaints about paywalls to the thread, which are guaranteed to be off-topic.
It's not a status quo anyone's happy with, and we'd be happy to hear suggestions of a better way, but please read https://news.ycombinator.com/item?id=8634076 before you do.