[voidbert]

I don't know about singular posts, but if you wish to delete your account and all your data, they must comply.

[londons_explore]

They only need to delete personally identifying data. That means they could probably just remove the username from your post, but leave the post text there attributed to "anon".

[Izkata]

They already do this, the username becomes "[deleted]"

[jsheard]

What about the third parties that Reddit has already sold the data to? Does GDPR require Reddit to chase them down and make them delete it as well?

[londons_explore]

yes, and if they cannot, they are violating the GDPR and have to pay fines.

This is why datasets are rarely sold as massive CSV's in the modern world - instead companies with data sell access to API's, with the expectation you'll query for what you need in real time.