According to this decent myth-busting listicle from the Smithsonian, the Spanish flu was not a “super virus” and may have been lethal as much because of the social context as any biological properties it had.
On the other hand … it was sequenced a few years ago and inflicted on monkeys that had some lethal immune system overreactions to it (“cytokine storm”). So that may account for why it killed so many more younger people than flus usually do. Seems like there was something a bit super about it. 😬