Earlier quoted context omitted.
HN allows everyone with sufficient karma to vouch for dead comments (or flag comments), I suspect most of the comment-level moderation you see is crowdsourced to fellow commenters; a still-dead comment means most of those who see choose to keep it dead. HN is awesome because of the rules and moderation (including bans); any unmoderated forum devolves into a cesspit; and it only takes a surprisingly few bad apples to…
That's the gospel, for sure. However, look at the dead comments here and, for each, tell us why it would turn HN into a "cesspit."
In many cases it's not the particular comment, it's the particular poster who is shadow-banned, and all of their comments are dead on arrival (to everyone but themselves, the definition of shadow banned). But people with showdead=true and enough karma can vouch for them to resurrect them if they're worthwhile.