The evidence seems to me to be doubtful with regard to establishing whether air travel carries any significant increase in risk for DVT particularly by comparison with other forms of travel.
However even if it does, surely the airlines can only be held liable during the period in which they should reasonably have been expected to know of the risk, (if there was any) and if they failed to make any evidence about that risk available to passengers.
Once the matter is out in the public domain, it is for passengers to choose whether they accept the possible increased risk and if they do, they accept the consequences of exercising their right to choose.
Surely a passenger who suffered from a DVT would, in addition to establishing causation, have to show that had he been aware of the evidence available at the material time, he would not have flown.
Given that the evidence is in dispute, and clearly out in the public domain, how many potential passengers decline air travel on that basis? Very few I suspect.