Ycombinator is a California business, and may be subject to CCPA/CRPA laws or those like them, possibly conditional on whether the account-holder is a California resident or not. Similarly, if the account-holder is an EU resident, I would imagine they have certain rights that they could seek to have exercised. Notably, you may or may not have the right to demand wholesale deletion of user-generated content, which is necessarily distinct from YC-managed PII, which for example might include your account's private email address. In all of the above cases, YC is likely within their rights to require you provide your legal identity, legal address of residence, and/or proof of citizenship of the protected region applicable.
However, if you can highlight specific IRL information that is considered Protected under those laws even though you voluntarily submitted it as UGC, such as your legal name, then you should request having that specific Protected information redacted within a given comment, but you'll need to be specific about which characters in which comments need to be redacted – a regular expression / expectation of work shifted to YC is an incomplete request in that regard. Were you to make a complete and specific request of specific redactions to be made to specific comments — I recommend using a double-quoted tab-delimited file with three columns: comment ID, unmodified comment, all redactions fully applied comment, and plaintext human description of redaction(s) performed — then I expect your redaction request would be evaluated by the site administrators without requiring any invocation of legal counsel or threats. Note that this is not a suggestion that you "replace entire comment with ~~~", this is a suggestion that you "replace the specific characters of PII in your comment with ~~~, leaving the rest of your comment intact". Note that I reviewed your past ~40 days of public comments and I see zero instances of possibly-protected PII.
If at any point an attempt to exercise legally-permitted rights is refused, then your only recourse is to hire a lawyer or seek one be hired on your behalf by an organization such as the EFF. You should probably be seeking legal counsel rather than posting for free legal advice on an internet forum, and I expect the first thing a lawyer is going to think (whether they say it or not) when presented with your posts and comments is to wonder why on earth you did not seek legal counsel sooner.
I empathize with your concern, but having lived through Dejanews doing this twenty-five years ago to all of Usenet (look up "X-No-Archive: Yes" for how the community reacted) and then having to get my old posts purged from Google's dataset purchased from Dejanews, Your realization about what you should and should not share publicly is one that Usenet as a whole pieced together decades ago, but was forgotten in the current generation's rush to participate in social media. Welcome to the "all my words for all time are searchable and no one will purge them from their full-text search indexes" crisis of thirty years ago, metastasized into individual sites rather than one big Usenet conglomerate feed.
(I am not your lawyer, this is not legal advice. I am not associated with YC or HN.)