It has not been a good year for the world, but we have achieved one remarkable and good thing. Flu infections are at historic lows. We have successfully suppressed the flu virus. We have been less successful with the, more infectious, SARS-CoV-2 virus that causes COVID-19, but with flu we have been successful. Presumably this is simply because these two diseases, flu and COVID-19, spread in very similar ways.
So, the rather indiscriminate measures we have used to combat spread of COVID-19, such as social distancing, quarantining, etc*, have strongly suppressed flu. As flu is less contagious (has a lower basic R number) it is more strongly suppressed by these measures than COVID-19 is.
This a good thing – flu has a much lower mortality rate than COVID-19 but is still unpleasant. It is also an interesting result from what is in effect an experiment that spans the world. We have conducted a huge experiment on the spread of all respiratory diseases, not just COVID-19.
We have done this by imposing worldwide (but variable) social distancing, mask wearing, quarantining etc. As the FiveThirtyEight article comments, some respiratory diseases seem much more affected than others by the measures we have taken to combat COVID-19. Presumably, this is telling us that some diseases like flu spread in very similar ways to SARS-CoV-2 while others spread in rather different ways. This is probably rather useful information for those studying flu transmission.
* Targeted measures such as track and trace only suppress COVID-19 not other respiratory diseases.