In what way?

You cannot delete your comments.

Are public comments in a public forum classed as private data under GDPR?

for example the only thing that can really be classed as PII is my username. does it count as reasonable to request it be deleted?

Yes and yes. Google “right to be forgotten”

Remember that GDPR is about storage and processing of personal information, not data created by a user. They are related by not 1:1 linked.

If the username is removed, and there is no reasonable way to link the user to the comment, then its not PII. I would hope that this is logical because its not personally identifiable. (caveats apply here like if you put your home address in every comment. However is it reasonable to expect a user to do that in a public forum? probably not. )

As you can request that your username is deleted here, and assuming they are deleted properly, then HN is reasonably following the user's request. Hence my assertion that HN is GDPR compliant enough to no worry.

The right to be forgotten is not an absolute right.