It has always been a good idea to mask up when you, personally, are sick. The West has only just learned this, while Japan has known it since the Kansas Flu. Even today, people do still sometimes come down with colds and the like, so you should still see some masked people (not always the same masked people, but you might not know that if you’re only seeing a person once).
The difference with covid was that covid has an extremely high rate of people having it while being completely asymptomatic. So when covid was epidemic, everyone was someone who might have covid, and so it made sense for everyone to mask.