There's no option in the interface, the best option right now is asking nicely per email.
Of course, you can just wait until May and then use the rights the GDPR grants you to force them to delete your comments.
It will/would apply to my posts - or my relationship with ycombinator, for example.
And then it would depend if/how yc would comply; by refusing access from the EU, or aiming for compliance.
I'm not sure how that would work.
Any compliant service is likely to allow self-service (eg: a button to delete a comment; a link to list out all data; an edit function to correct wrong data).
If you're storing personal information and don't comply with the law, you risk a fine. Just as you risk a fine for mismanaging health data, or risk prosecution for storing data that is illegal, like child pornography.
"Information provided under Articles 13 and 14 and any communication and any actions taken under Articles 15 to 22 and 34 shall be provided free of charge. Where requests from a data subject are manifestly unfounded or excessive, in particular because of their repetitive character, the controller may either:
charge a reasonable fee taking into account the administrative costs of providing the information or communication or taking the action requested; or
refuse to act on the request.
The controller shall bear the burden of demonstrating the manifestly unfounded or excessive character of the request."
This is grossly oversimplified to the point of almost being wrong, but is the general idea of what applies here.
There is countless precedent of the US using these tactics to enforce IP law, and EU countries using them to enforce consumer laws.
It is expected that the EU would do exactly this.
For example, if YCombinator refuses to adhere to the law, but holds shares in an EU startup (and they do in several), then those shares could be seized and auctioned off to enforce the law.
PS: how does one delete old HN comments? Seems like a major privacy issue to me.