And when this is combined with Facebook/Google/Apple/Microsoft shadow profiles, it's an almost certain guarantee that information is linked with your name, DOB, credit info, and tax ID number.

Not disagreeing with you or parent in general, that a post (or as you point out a history of posts) can be personal data.

What I'm saying is, in general it isn't. If you sign up to platform XYZ and make a post, in general, the content of the post is not personal data. It certainly can be. But by default it isn't. That's a misconception I want to dispel.

Yep I agree, if you posted on a specific subreddit this is already personal information about yourself. There is no way they are not violating GDPR if they deny a request to delete (but I guess they can block direct deletion in bad faith and tell you to submit a direct request).

I'm not saying the content of a post can not be personal data. I'm saying it is not by default.

The words "Hello world" posted on any subreddit is not personal data. In this case the company could simply delete the information relating to you such as user name, IP etc.

The content you post is not, in general, personal data. It certainly can be.

That's kind of funny, because "Hello World" gives a strong indication of your profession and interests. Not enough on its own to be PII, but a useful data point.

The moment they post it publicly, it is no longer considered personal private content

That’s not the definition of personal data.

I can post my name publicly, but it’s still considered personal data. It doesn’t magically become a free for all just because it’s out there.