Starting from the end
> Can I request all of this information be deleted?
Yes, the ones that are on HN. I'm not sure how it works for 3rd parties that obtain your data
> does this include other comments that referenced my username?
I don't think so, this is unlikely, especially as you didn't create it and HN doesn't link this (as opposed to reddit)
> What about comments that might have linked to my GitHub profile?
I'd say that being required is even less likely as HN has no way of knowing what's your GH profile
GDPR is what they know about you. If they're actively trying to link pasted GH profiles and usernames then this would apply, otherwise no.
> What if there are server logs that include my IP and a time which can correlate to when I posted a comment or something?
That thing with IPs being PII I'd say this would apply, but then again, this doesn't bring any new information.
So if they keep track of users access times then yes, but if this information is rotated, sent to /dev/null then no.
You're not obligated to connect all the dots, or track user login times. That being said, IP (especially + times) are PII so better anonymize it and discard once not needed.