You’d be surprised how big players work around those.
I asked GitHub to remove an issue from a repo whose owner blocked me. Being both I and the owner EU users, I sent a GDPR removal request. They just said they’re a “controller” and that the request would be forwarded to the owner.
Nothing came of it.
GitHub even has customer support, Reddit does not, so you can imagine how little chances you have in doing so unless you fire up your lawyers.
The more recent GDPR on the other hand should allow me to ask the "owner" (I forgot the exact name) to delete all the data related to me. He however declined to follow my request. An option at that point was to pay something like €50 to file a complaint to the governing body (some EU entity), so I gave up.