"Hellbanning" is a form of banning where the site takes steps to avoid tipping you off that you've been banned. It's ostensibly a defense against spammers and trolls, who will create new accounts after being banned.
For the past couple years, bans on HN are announced publicly. They're not automatic; the moderators do it directly. It's not based on your karma.
For the most part. We don't do so for spammers or (sometimes) serial trolls.
Top-level comments that are literally seconds old are typically posted about 2/3 down the page. None of my votes appear to actually count. It's been this way for several months.
I'm used to it, but it's still very annoying and I wish HN would actually acknowledge they do this.