As University of Alabama student did indeed die doing the exercise perhaps 7-8 years ago. The instructor was SSI. The exercise was not then (and is not now) part of the SSI curriculum, so the instructor added it on her own. She was not physically supervising the incident when it happened. I never heard the results of the lawsuit, but I imagine the insurance company settled very high and very fast.
A couple decades ago a landmark study of student incidents during training revealed that breath holding on ascent, leading to embolism, was the number one cause of student injuries and fatalities. The prime culprit was the CESA. As a result, PADI and most other agencies adopted extremely strict rules on how the CESA must be done, with the instructor very much in control of the situation at all times and ready to intervene if the student is detected holding the breath. I do not know for sure, but I suspect that is when the doff and don left most instructional practices. The student does not have to hold the breath long during an ascent to cause a problem.
I certainly don't know the rules of all agencies, but for most of the mainstream ones, if the instructor is doing the doff and don, it is in addition to the curriculum and against the wishes of the agency. If there were to be an accident, the instructor would have a tough time defending it in court.