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I can't imagine the DB load of hundreds of thousands of people deleting all their comments. https://shreddit.com/

It's very likely that a deletion at reddit is just setting "is_deleted" to true in a row, not actually a proper DB removal.

What about rebuilding the caches for all those years old threads?

correct, if you do the data export for your account you get all of the deleted comments/submissions in the resulting zip.