Something to be considered is that today's population is on average older than in 1968 and therefore there are more people vulnerable to respiratory illnesses than ever(*atleast before COVID19).
It was pretty bad. The flu has all the long term complications for some people that covid does - the lung problems, heart swelling, brain problems, etc.
I talk to people who were alive in 1968. It just wasn't really on anyone's radar. Rarely made headlines in the U.S.