What would happen if the Earth's axial tilt were to decrease from 23.5 degrees to 21.5 degrees?

The most immediate effect would be a fast expansion of the north pole ice cap and the freezing to the ocean surrounding Antarctica.

In the northern hemisphere there is about a 1000 mile zone starting at just below the polar circle and extending about 1000 miles southward where most of the earth's conifer forests exist. This zone is responsible for a very large portion of the oxygen production for the earth. By changing the angle 2 degrees the conifers would have to shift southward which might not be possible because of the vegetation existing there now. That is, the broad leaf hardwood forests of the northern hemisphere which also plays a huge role in the earth's oxygen production.

Like everything, trees exist in the climate which best suits them. That means the broad leaf trees at the northern portion of their reach would likely die and create a treeless wasteland.

Because the sun would shine directly over the area from 21.5N to 21.5S, the average temperature would rise and cause certain deserts to expand beyond their present borders.

At all latitudes there is vegetation and animal life that cannot survive any large shift in their general environment which would lead to their extinction. Great extinctions of the kinds this shift would cause is impossible to predict except to say that all life on earth would be adversely affected.