Drawing the line

It's been a while.
About this time in 2019 we saw this phenomenon so often we didn't even register it (not me anyway) but in 2020, the straight lines drawn over the sky on a clear day, stopped gradually, until I noticed that I hardly ever saw them. Now they are slowly coming back and the industry (Ryanair) predicts they will soon be back to at least the same number of flights as before the pandemic.
Personally, I doubt it.
If you wonder who is drawing the line, check out this page to spot the plane.