WHO changed its position on masks as late as June 2020. https://www.webmd.com/lung/news/20200608/who-changes-stance-...
The US government changed its stance by April: https://www.dallasnews.com/news/public-health/2020/04/08/why...
In much of the world, people who wore masks were subject to ridicule, especially on social media because of the cynical public health messaging, which seemed to be about preserving stocks of masks for healthcare workers by telling the public that “masks don’t work” (https://www.nytimes.com/2020/03/17/opinion/coronavirus-face-...).
The New York Times op-ed above, and efforts from the Czech Republic made discussing public masking more acceptable in the US, and then the rest of the world.