From my post history on Reddit, you'd know where I live, my age, gender, job, relationship status and a good amount about places I've been and my interests. There are already websites that collate that info from a profile to give a summary of the user. Under GDPR, that quite possibly adds up to personal data relating to an identifiable person.
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.
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.
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.