Most deaths were actually caused by bacterial pneumonia rather than the flu virus alone.
Fauci’s 2008 paper which reviewed the Spanish flu determined most people died from pneumonia. The conclusion made in the 2008 paper is that America should stockpile antibiotics in case there is another viral epidemic, since people die from secondary bacterial infections. There was no mention of masks.
They did not have penicillin or any other antibiotics in 1918. Many people, especially the old, died from bacterial pneumonia back then.