Obviously, the massive reduction of air traffic is one of the most striking impacts of the Covid measures, and it has a significant positive effect on the environment.
EUROCONTROL, the European air traffic control organisation, has a very interesting section on air traffic and Covid19, including visual before-after comparisons (like the one shown here).
There is a network of 150+ initiatives, Stay grounded!, which is campaigning for #SavePeopleNotPlanes.
21 April 2020: Austrian Airlines announces a downsizing of its fleet. A maximum of 75 percent of the pre-coronavirus level is expected by the end of 2021. Austrian Airlines Executive Board member Andreas Otto comments: “The entire airline industry is pessimistic. We have to assume that we will reach the ‘pre-corona level’ again in 2023 at the earliest.”