They definitely do, it’s common for such systems to never actually delete anything because storage is cheap. It likely just is flagged deleted=true and the searches just return WHERE [post].Deleted = False on queries on the backend.

So it looks deleted to the consumer, but it’s all saved and squirreled away on the backend.

It’s good to keep all this shit for both legal reasons (if someone posts illegal stuff then deletes it, you still can give it to the feds), as well as auditing (mods can’t just delete stuff to cover it up, the original still exists and admins can see it)