The domain is visible next to the submission. Tags like [video] and [pdf] are useful when it isn't completely obvious but the WSJ domain is already strong enough of an indication to check the comments for alternatives imho if that's something you need.

That wouldn't be appropriate because articles with hard paywalls aren't allowed on HN.

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:

https://news.ycombinator.com/newsfaq.html

Yep, that convention could have really worked.