The To: address in the header would be interesting. Of course, you wouldn’t want to disclose it verbatim here but it might be useful to have a rough idea. Was it [email protected] or some variation of that, or was it more like [email protected]? Some people here think it doesn’t matter, that it’s inherently personal info, but the European Commission says it matters. It’s not hard and fast; there are varying shades of gray here. Maybe they kept logs of your IP address and maybe that makes a difference. You might want to read WP136 (I have yet to read that).
I would love to see action taken against Reddit, if anything just to burden their lawyers and create some costs for them. But I doubt it will go anywhere. GDPR enforcement is such a shit-show in Europe. Even dealing with clearly blatant violations that are wholly internal to Europe which should irrefutably incur penalties, simple obvious cases are being ignored by DPAs. So I have little confidence that this cross-border case against a non-EU data controller would actually get results when the law is not really concrete. The one factor in your favor is that Reddit is somewhat high-profile which might take a DPA’s interest.
I don’t think a “delete my account” button constitutes an Article 17 request. It removes the purpose of processing to some extent, which then relies on the data minimization principle (Art.5). Reddit can do a bit of hand-waving to make excuses like needing to retain your email address in case one of your posts sparks a legal inquiry. Your case would be stronger if you had submitted an explicit Art.17 request to Reddit.
From the email:
Per our lawyercats, we are not able to respond to further inquiries or questions.
I wonder if that statement might be actionable. Art.12 and 13 require Reddit to identify a data controller with a point of contact and to tell you your GDPR rights (IIUC). And here they are outright stating in effect “we don’t want to hear from you”. I would stress that in your GDPR complaint, not just the misuse of your email which you expected to be deleted. But note they do provide an address at the bottom of that msg. Although that angle of attack might require Reddit having a way to know you have ties to a GDPR region after the supposedly “deleted” your acct.
Also, I would look into any anti-spam laws your country has. There may be a higher degree of legal actionability there.