By 2019, they knew science didn't back masks

Post #1098

When looking back in time by using the Wayback Machine (archive.org), you can see the 2019 respiratory virus guidance for non-pharmaceutical interventions (NPIs) from the World Health Organization:

But it turns out that, in 2019, the WHO knew that the science did not support the use of face-masks for any kind of an attempt to slow the spread of a respiratory virus:

Here is the meta-analysis on masks against influenza:

Reference

[2019 advice on masks] — https://web.archive.org/web/20240927035345/https://iris.who.int/bitstream/handle/10665/329439/WHO-WHE-IHM-GIP-2019.1-eng.pdf