You can be reasonably certain they didn’t call on people to stop going to shul and to stop learning Torah.
One should also note that unlike Covid19, Spanish flu killed people of all ages, and most people infected became quite sick. Spanish flu wasn’t reported in Anglo-American media, at first, since it was considered actionable intelligence of the health of the armies – it became public knowledge when it was reported in the Spanish press, since Spain was neutral and therefore had no military censorship. It them became well known everywhere. In the USA, many people wore masks and many things were cancelled, but no one attempted to shut down the economy.
The infamous cholera epidemic cited above shows the problem with rabbinim paying too much attention to doctors. In turned out the disease was not an airborne virus, but was spread by polluted drinking water. Any quarantine measures were not warranted, and fasting would actually reduce your chance of exposure. The remedy was to acquire clean water for drinking, washing and preparing food (e.g. boil the water before use). It turns out the doctors the rabbis consulted with were into what we now call “junk science”.