It's impossible to know how prevalent it is. I'm not even sure of how to quantify it? I'd read about it going back many years, I was aware of it in the 90s, but only from newspaper reports. I'd generally assumed it was mostly somewhat of an urban myth, exaggerated, too much drink, etc.
However, I'd had the experience a few years back, as a mature student in my early 40s. Two of the mature students, somewhat reluctantly going to drinks with the main group, 2 glasses of red wine for me. I was getting a train afterwards, and no intention of drinking lots with younger students, etc.
I lost contact with the group I was with, but luckily some other people from the course noticed me, and managed to get me to their place. It's a long story, but they eventually (I'd lost my phone/bag) managed to contact DH who collected me around 4AM. I do remember bits, I got sick several times. I insisted that DH take me straight home. I was very very anxious for several days afterwards since I'd had to change some clothes, and didn't know what happened. I did make a report (a few weeks later) to the police, and to the venue.
My awful awful experience makes me believe it is fairly rare, otherwise this would be something I'd certainly have heard directly from friends if they had experienced it.
It also makes me really concerned that there are people out there that would do this. It is highly dangerous.