Yes in boxing kidney punches are illegal and for good reason as getting punched in the liver is extremely painful and can be fatal.
So if you do a kidney punch when boxing someone then you could potentially kill that person or injure the person severely with a kidney punch.
A kidney punch can rupture a persons kidney which is similar to having a splenic rupture in that it is usually caused by a direct blow to the abdomen, side, or mid to low back which causes damage or a tear to the organ or kidney.
So by damaging the kidney it can lead to kidney failure and eventually more kidney problems and possibly death if not treated.
So doing a kidney punch in boxing would get your removed from the boxing game as kidney punches are indeed illegal in boxing.