All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
We have not found any statement from the contemporary scholars regarding the training corset, but wearing this evidently does not involve the forbidden change of Allaah’s Creation, as it does not involve any surgical operation such as severing part of the body, as with filing the teeth (for the purpose of beautification), which is forbidden. Rather, it is like sports training and natural materials that dissolve fat. Fat is not an original part of the human body because it vanishes with diet, exercise and natural fat-burning agents.
The Islamic Fiqh Council stated that what is forbidden without necessity is performing slimming surgeries such as liposuction, and there is a difference between this and wearing the training corset. A resolution from the Islamic Fiqh Council reads: “It is permissible to reduce weight (slimming) with standard scientific methods, including surgery (liposuction) if the weight is considered a medical condition and there is no other method than surgery, provided one is safe from harm.”
It can be understood from this statement that any means of "slimming" other than surgery is permissible even without a medical condition, so it includes wearing the training corset.
Allaah Knows best.