If it's on the front page of HN, it's a signal that the article can be accessed by anyone.
It may need a workaround, which someone will post in the comments.
If no workaround exists (e.g. using http://archive.is/ or https://outline.com/), the article should be flagged and removed from the front page.
The FAQ covers this:
It is answered in the FAQ  and re-iterated frequently by dang.
Paywalled pages are OK if a bypass is available. If there is one, usually someone will post it in the comments.
If nobody has yet, you could try viewing the article in http://archive.is/ or https://outline.com/, and if it works, share the link here.
If no paywall bypass is available, the item should not be on HN, so you should flag it.
I know the status quo seems imperfect; it absolutely is, but thoughtful consideration always leads to the conclusion that the status quo is the least-worst of the available options.
The quality of the discussion may also reduce, as the non-subscribers can only respond to commenters without benefit of context of the original material.
Should enough of these posts exist, the effects could impact the community at-large; resulting in a decrease in diversity of opinion, and the popularity and relevance of HN as a site. This, in turn, could give rise to, or cause a traffic shift towards, more open competitors.
> I frustrate when I clicked to a title or entry and when I go to the tab I see that I need to pay for a subscription to see the content. I think this creates a great struggle in an open community like HN.