Cities like San Francisco took that advice to the next level, even writing a law around it. If a person was caught without a mask in public or even wearing it improperly, they were arrested or fined.
San Francisco was lauded for its proactive response to the virus, but city officials relaxed their restrictions following the fall of 1918. After the third wave in the spring of 1919, San Francisco ended up with some of the highest death rates of the flu in the US.
Source: CBS News