[jsty]

IANAL

My armchair argument would be that when you sign up to post on a public forum, you should have every expectation that your posts will remain visible. Expecting otherwise would be rather like publishing a book before invoking GDPR to 'un-publish' it and demanding to have all the sold copies destroyed several months after release simply because your name is on the cover.

The expectation of content remaining public is also explicitly stated in HN's terms, along with a pretty broad-ranging agreement to allow them to use the content you upload. Thus arguably HN has a contractual right to continue publishing the content, as allowed for under art 6.1(b).

GDPR Article 6: "Processing shall be legal if and only if to the extent that at least one of the following applies ... (b) processing is necessary for the performance of a contract to which the data subject is party ..."