HN probably doesn't fall within the material scope of GDPR, unless they perform business activity that falls within the scope of EU law that I'm not aware of.
That would be different if they marketed/promoted/sold in the EU, offered European language or currency support, or somehow otherwise took action to position themselves for the EU.
As a thought experiment, if HN was regulated by GDPR:
1. Yes, all kinds of user generated content can contain GDPR Art. 9's special categories of personal data. HN would probably rely on the exemption in Art. 9(2)(e), which permits processing "personal data which are manifestly made public by the data subject." The purpose of HN is to let you share your own data on the Internet, that's the entire point. That's fine under GDPR.
2. HN would still need a lawful basis for processing under Art. 6. For a paid service, a Terms of Service would normally be fine. I don't think HN has or wants one of those, and they don't track users at all before registration, so they could collect an explicit consent from users on registration. If they did track prior, a cookie popup could collect the consent. Also, under Art. 8, the default minimum age of consent is 16, so we'd want to consider age confirmation too.
3. Archiving posts on the Internet forever is not a problem, if that's the intended use of the site, which it is. My guess is that deleting a user and their posts is feasible at the application/database layer. The problem would be deleting personal data from backups of the site if the user withdraws their consent and requests Art. 17 erasure. In that case, only retaining the backups as long as necessary and documenting that justification internally is probably sufficient.
4. Article 22 restricts "automated processing, including profiling, which produces legal effects concerning [the data subject] or similarly significantly affects" the data subject. Ranking, voting, and anti-spam probably don't qualify as weighty enough subjects to be restricted. Recital 71 ("Profiling" https://gdpr-info.eu/recitals/no-71/) sheds some light on what the EU is trying to prevent.
5. They'd have to get a data protection agreement or other Art. 46 agreement with hosting vendors. Cloudflare is on top of this: https://www.cloudflare.com/gdpr/introduction/ Not sure what other subprocessors are involved.
6. Being able to see most of your own data on HN means you have Art. 15 access, which is nice. I think they'd have to also give you any hidden metadata as well. Not sure what that might be (vote weight score?).
6. There's a bunch of other stuff they'd probably do, like appoint a data protection officer, publish a privacy policy, add the ability to delete your account, etc.