Square shape breasts after reduction? How likely?

Over time, the square shape bottoms out... in the long term, the square shape initially is a good thing.

Breast reduction surgery tends to be one of the most patient pleasing operations we perform; obviously, the outcome of the procedure will depend greatly on the plastic surgeon who performs the procedure. Therefore, it will be important for you to select your plastic surgeon carefully. Ask to see lots of examples of his/her work achieving the types of outcomes you will be pleased with. Then, communicate your goals/questions/concerns carefully as well. Best wishes.

Usually the square occurs due to the breast being lifted up the chest wall and over time the breast will drop into the scars and give a more aesthetic appearance. Best to look through images with your PS who can show you how results change with time.