I have blacklisted reddit.com on my DNS, but I will keep an eye out if my comments are restored. I deleted thousands of comments, all of them, earlier this month.
And then what? I looked at the request form and it's pretty clear that they've automated those as a regular batch job, so other than a spike on some graph they're unlikely to notice that.
And they will get a reasonable answer that reddit doesn't store personal information beyond what it needs to operate.
It's technically impossible to identify correct personal information in users' posts.
I don't see how they can argue that they own the data, and then turn around and say they have no way of making sure there is no PII - if a user thinks that a comment or post contains their PII and delete it, then either the user owns it or reddit needs to respect that is was deleted for that reason
Post something personal, then put in a 1st GDPR request and ask for all the servers (and their jurisdictions), all the backups, all the 3rd parties with access related to the post itself. Then ask for the security certifications and GDPR policies of each entity named. Then ask the company to delete the post for you in a 3rd GDPR request.