Negatives that are stored flat tend to remain that way. Freshly developed negatives curl. Flat storage eventually cures some of that. Humidity also affects curl.
Some negatives from the 1970s did curl. Do you remember the notorious Kodak 2475 recording film?
Yes, all my older negatives are flat also. So are my newer negatives after a couple of weeks in sleeves. You have to store the negatives flat for them to achieve flatness. Under a book for a while is good.