Someone posted not too long ago that a person who was part of Twitter’s group over the GDPR - pre Musk - said the lawyers came to the conclusion that tweets were protected under the GDPR.
I believe it's less straightforward than that. Under GDPR, consent can be withdrawn, you can't give an irrevokable consent.
Presumably falls under right to erasure (art 17,19 of GDPR). You've withdrawn your consent, so if it isn't exempt under legal obligation, public health, scientific research etc then that's it, really. I guess there might be brave souls who argue that posts on Reddit sometimes don't qualify as or contain personal data, but that would seem irrelevant unless someone is painstakingly anonymising the dataset on a case by case basis, which they surely aren't.
Surely, if twitter DMs fall under the GDPR, so do Reddit posts and comments (and note that it's the content of the DMs, and not the personal identifiers, and that the DMs are requested to be deleted from e.g. receipients inboxes as well).