How windshear affects IAS:
Imagine model plane flying steadily with zero groundspeed, 20 kt IAS in front of a fan.
Fan is suddenly turned off.
Model plane is suddenly at zero airspeed, will lose height rapidly and either crash or accelerate to a suitable airspeed to fly away before it hits the floor.
Not the same as the downwind turn in steady strong wind case - as succinctly described above, aircraft flies with reference to the air and, to the pilot in IMC, will feel the same if a constant IAS turn is flown no matter what the steady wind.
In visual low level manoueuvering, the visual slip/skid illusions fool us into thinking the airspeed changes are an effect of the wind.