Has recent drop in airline flights over the US had a measurable warming effect as was found after 9/11?
It's not clear yet; this is an area of active research which it itself hampered by COVID-19 social distancing requirements, according to this report on Climate Change News:


  Scientists with Nasa and European research groups hope to use clear
  skies to narrow down massive uncertainties about the warming effect of
  condensation trails – the wispy white lines that criss-cross the skies
  in the wake of jets engines.
  
  “The air traffic system has not been diminished to the current levels
  since the days following 9/11,” said Patrick Minnis of Nasa Langley
  Research Center, who is joining a research effort to study
  high-altitude clouds.
  
  “Flight groundings at the scales initiated in response to the
  coronavirus pandemic are a significant opportunity to better quantify
  the impact of air traffic on cloud cover via contrail formation.”


As to the post-9/11 effect:


  The new research builds on studies after the 9/11 suicide hijackings
  in the US grounded flights for a few days. One study, for instance,
  found that the plane-free skies had an impact on temperature
  variations in the United States, but some researchers say the findings
  might have been caused by natural variations.