Paywalls vary depending on region, besides which people disagree about how to use that word, so such an indicator would generate a lot of disagreement.
There will always be disagreement, but this is a very low-effort feature that brings value to users (it seems based on these small poll numbers)
It could be as simple as a single ASCII character appearing before or after the story title. You could have a checkbox on the Submit Story page that says "Paywall?" to make it appear. People could continue to check the comments for the paywall workarounds as they do today. But they could also skip articles that the submitter thought was a paywall.
HN's approach is that if there's a workaround, it's ok. Users usually post workarounds in the thread. This is in the FAQ at https://news.ycombinator.com/newsfaq.html and there's more explanation here: https://hn.algolia.com/?dateRange=all&page=0&prefix=false&so...
This was established a long time ago: https://news.ycombinator.com/item?id=10178989