Deep Vein Thrombosis from sitting in airplanes is real. It happened to me in 1963. I got a clot in right leg below knee on DC 8 flight from Honolulu to Miami.
I now have chronic venous insufficiency.
I also got clots over the years from sitting in cars, behind the yoke of my Mooney, and in front of computers.
I sue nobody. I wear elastic stockings. I take Coumadin. I exercise.
The problem is stasis of blood in lower extremities for hours.
The solution is exercise during sitting to get blood flowing. It is a solvable problem in flght by flexing calves. Another solution is to elevate the legs above heart.
As usual, education is the answer. To implement is difficult.
Every pilot is at risk. Flex those calves.
Cheers,
JohnBarrySmith (GIB, guy in back)