This thread is so over the top.
GDPR is intended to protect individuals from companies processing and misusing user's personal data. This includes traditional PII as well as cookies and IP addresses. Upon request, you can have these items shared with you and / or purged from a company's database.
As has been mentioned above, GH is run on top of a pretty standard forum software package. This package collects WHAT YOU GIVE IT. The only items they're collecting which you have not entered yourself would be the IP address used to access your account and any 3rd party tracking that might be on the site. As you can see with this Ghostery scan (Chrome plugin), the only 3rd party tracking that this website has deployed is Google Analytics, which does not collect personally identifiable information (PII). Its job is to understand site performance. Where people are coming from, how long they stay on the site, etc. This information is anonymous and aggregated (meaning you can't reverse engineer to tie to an individual) for all GA accounts.

- Can this website delete your account and post history? Yes. It's an option in the admin panel
- Should you suddenly feel differently about having an account here now that MD owns it? That's for each to decide, but I can't think of a reason why you're at more 'risk' now that MD owns the site. And if that's the case, I'd also advise you to close your Facebook, Twitter, Snapchat and every other internet account you have (as they ACTUALLY use your information to sell advertising)
- Will the EU government fine MD over your GDPR requests? Please. Not even close. This website doesn't collect your personal information (beyond IP address) and does nothing with it. But if you feel that you're at risk, I suppose you could reach out to a GH admin to delete your account.
Source: 10+ years in digital media analytics, targeting and optimization