I think that it is an "automod" situation, since most of the articles are [dead] without being [flagged]. Sometimes, a reader will vouch for a [dead] article which brings it back to life.
Is there a public list of blacklisted domains?
I don't know.
Personally, I don't object to this behavior with regards to 404 because most of their articles aren't good and/or on-topic enough to be useful. The ones that are often get vouched for and appear anyway. From where I sit, this seems like the least-worst result.
The answer is that paywalls are allowed when there are workarounds (such as archive links) which allow ordinary readers to read the article without paying or subscribing, while hardwalled domains (i.e., without such workarounds) are banned.