1. As a pilot defending against GVs attacking his airfield I had the impression that M4 is the most popular tank. The stats tell otherwise. Perhaps they are used mostly for attacking bases?
For the field itself I have seen much more T-34/85 than any other tank (which makes sense), but that's about impossible to quantify.
M4(75) with rockets are still very popular for attacking town. The reason they do not appear more on the chart is them being eliminated by planes much more than the other tanks.
2. Why did the tank destroyers disappeared with the arrival of the new terrains? Is that because of shorter free lines of sight?
Yes. The German TD are mostly standoff weapons needing long ranges with unobstructed view and little risk of being flanked. The new terrains have very few spots like this. Now it's all about maneuverability and short range firepower instead of optics and long range precision.
Also 'concrete sitting' on VBases has been greatly reduced (IMHO generally a good thing), which was often used by the Jagdpanther (and the Tiger II, of course)
3 Do wirbs and ostwinds pose any danger to tanks at short ranges?
Beyond tracking, not that much.