So I don't think anyone's replied yet who has actually had a DVT? I have, about 4 years ago. I'm a fit guy (marathon runner) in mid 40's, so was a bit of a surprise after a relatively short flight (5hrs). Several months on anti-coagulants and it resolved fully, thankfully.
Many people get their legs swelling up during flights - this is not a DVT, generally just fluid retention.
Symptoms of DVT usually occur a while later - swelling, pain, red skin (as blood flows through surface capillaries/veins rather than main veins).
I had no swelling, no redness. Just woke up in pain a couple of weeks after flight feeling like foot was about to explode. Thankfully got it checked out and diagnosed quickly.
If you are not aware, and brush it off, you can die. Many people die of complications after DVT, and it is thought many are never diagnosed. Nobody really knows how many people are effected, since only those who present to doctors are counted of course.
Went through the tests for genetic factors but all negative. And no other factors (not overweight, not a smoker, not taking the pill etc).
Am still flying (too much) - precautionary measures (according to two haematologists) are - keep moving - do those exercises!, keep hydrated, wear compression stockings, take aspirin before and during flight, stretch out if possible (Premium economy or business preferred!), anticoagulant injections are an option, but I've never taken them.
Having had a DVT, I am statistically more likely to get another than those who have not. But being aware of the risk, I believe the chances are now lower than for many other people. I hope!