A lot of people died in the Spanish flu. This is because people at the time did not:

Other options by popular demand

Spanish flu was an A/H1N1 flu. The A/H1N1 virus is now one of the seasonal flu viruses that circulate each winter.

Just as today with SARSCoV2, nobody died of the flu, everybody died of pneumonia. There is one common treatment for pneumonia today that only became available after WW2.

The secondary pneumonia that killed people during the Spanish flu was bacterial in nature.

Bacteria of course thrive on moist masks, and by breathing in through these masks you inhale them straight into your respiratory system. This happens with all masks, cloth or plastic. Petri dishes are made of plastic.

"The majority of deaths in the 1918-1919 influenza pandemic likely resulted directly from secondary bacterial pneumonia caused by COMMON upper respiratory-tract BACTERIA."
Anthony S. Fauci https://pubmed.ncbi.nlm.nih.gov/18710327/#:~:text=Conclusions%3A%20The%20majority%20of%20deaths,are%20consistent%20with%20these%20findings