This would be greatly realistic and possibly an easy addition.
Not greatly realistic if you got contrails every time you went above 20k. A few times a week, try to tear your gaze from the monitor and look out your window. See any contrails? You can be sure there are lots of airplanes 20k and above. Some days the contrails form or some days not. Some days they persist only briefly for a short distance behind the plane, other days they can go horizon to horizon.
The reason you see them so often in combat films is because, well they're photogenic. No sense filming yet another formation of airplanes, but as soon as dramatic contrails form, roll cameras.